The gun control measures a tearful President Barack Obama announced Tuesday would not have prevented the slaughters of 20 first-graders at Sandy Hook Elementary School in Newtown, Connecticut, or 14 county workers at a holiday party in San Bernardino, California.
Obama's executive action expands mandatory background checks to gun shows, flea markets and online sales, adds more than 230 examiners and staff to help process them and calls on states to submit accurate and updated criminal history data.
Those measures are seen as crucial to stemming gun suicides — the cause of two-thirds of gun deaths — by blocking immediate access to weapons. But, an Associated Press review shows, they would have had no impact in keeping weapons from the hands of suspects in several of the deadliest recent mass shootings that have spurred calls for tighter gun control.
The shooters at Sandy Hook and San Bernardino used weapons bought by others, shielding them from background checks. In other cases, the shooters legally bought guns.
In Aurora, Colorado, and at the Navy Yard in Washington, D.C., men undergoing mental health treatment were cleared to buy weapons because federal background checks looked to criminal histories and court-ordered commitments for signs of mental illness. The Obama administration is making changes in that realm by seeking to plug certain Social Security Administration data into the background check system and by helping states report more information about people barred from gun possession for mental health reasons.
The suspect in a shooting at a church in Charleston, South Carolina, should have been flagged at the time, but errors and delays cleared the way for his purchase.
See President Obama addressing the nation after mass shootings throughout his tenure:
Obama addressing the nation on U.S. shootings & gun control during his presidency
Obama measures wouldn't have kept guns from mass shooters
An emotional President Barack Obama, joined by Vice President Joe Biden, pauses as he recalled the 20 first-graders killed in 2012 at Sandy Hook Elementary School, while speaking in the East Room of the White House in Washington, Tuesday, Jan. 5, 2016, about steps his administration is taking to reduce gun violence. (AP Photo/Jacquelyn Martin)
US President Barack Obama gets emotional as he delivers a statement on executive actions to reduce gun violence on January 5, 2016 at the White House in Washington, DC. AFP PHOTO/JIM WATSON / AFP / JIM WATSON (Photo credit should read JIM WATSON/AFP/Getty Images)
US President Barack Obama delivers a statement on executive actions to reduce gun violence on January 5, 2016 at the White House in Washington, DC. AFP PHOTO/JIM WATSON / AFP / NICHOLAS KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)
President Barack Obama makes a statement on Wednesday's mass shooting in San Bernardino, Calif., Thursday, Dec. 3, 2015, in the Oval Office of the White House in Washington. (AP Photo/Evan Vucci)
President Barack Obama makes a statement on Wednesday's mass shooting in San Bernandino, Calif., Thursday, Dec. 3, 2015, in the Oval Office of the White House in Washington. (AP Photo/Evan Vucci)
President Barack Obama pauses as he speaks in the Brady Press Briefing Room at the White House in Washington, Thursday, Oct. 1, 2015, about the shooting at the community college in Oregon. The shooting happened at Umpqua Community College in Roseburg, Ore., about 180 miles south of Portland. (AP Photo/Susan Walsh)
President Barack Obama pauses as he speaks in the Brady Press Briefing Room at the White House in Washington, Thursday, Oct. 1, 2015, about the shooting at the community college in Oregon. The shooting happened at Umpqua Community College in Roseburg, Ore., about 180 miles south of Portland. (AP Photo/Andrew Harnik)
President Barack Obama, left, sits with FBI Director James Comey, right, before speaking to members of the media in the Oval Office of the White House in Washington, Thursday, July 16, 2015, on the shooting in Chattanooga, Tenn. A gunman unleashed a barrage of fire at a recruiting station and another U.S. military site, killing at least four Marines. (AP Photo/Pablo Martinez Monsivais)
President Barack Obama walks across the South Lawn of the White House in Washington, Thursday, July 16, 2015, upon his arrival on Marine One helicopter after a short trip from Andrews Air Force Base, Md. Obama was heading towards the Oval Office and there he would be briefed by his counterterrorism and homeland security adviser and the FBI director on the recent shooting in Chattanooga, Tenn. Obama promised a thorough and prompt investigation into an attack at two military sites that killed at least four Marines. (AP Photo/Pablo Martinez Monsivais)
President Barack Obama claps along to the music as he arrives with first lady Michelle Obama for services honoring the life of Rev. Clementa Pinckney, Friday, June 26, 2015, at the College of Charleston TD Arena in Charleston, S.C. Pinckney was one of the nine people killed in the shooting at Emanuel AME Church last week in Charleston. (AP Photo/Carolyn Kaster)
In this June 19, 2015, photo, President Barack Obama pauses as he speaks about gun violence at the Annual Meeting of the U.S. Conference of Mayors in San Francisco. Conceding that congressional action was unlikely soon, President Barack Obama said lawmakers will tighten federal firearms restrictions when they believe the public is demanding it. "I am not resigned," Obama said. "I have faith we will eventually do the right thing." (AP Photo/Carolyn Kaster)
President Barack Obama, accompanied by Vice President Joe Biden, pauses while speaking in the Brady Press Briefing Room of the White House in Washington, Thursday, June 18, 2015, on the church shooting in Charleston, S.C., prior to his departure to Los Angeles. (AP Photo/Susan Walsh)
President Barack Obama pauses while speaking in the Brady Press Briefing Room of the White House in Washington, Thursday, June 18, 2015, on the church shooting in Charleston, S.C., prior to his departure to Los Angeles. The current brick Gothic revival edifice, completed in 1891 to replace an earlier building heavily damaged in an earthquake, was a mandatory stop for the likes of Booker T. Washington and the Rev. Martin Luther King Jr. Still, Emanuel was not just a church for the black community. (AP Photo/Manuel Balce Ceneta)
President Barack Obama listens to a question during a Tumblr forum from the State Dining Room of the White House in Washington, Tuesday, June 10, 2014, moderated by Tumblr Founder and CEO David Karp, left. During the forum Obama conceded he was ashamed as an American and terrified as a parent that the United States can't find it in its soul to put a stop to rampant shooting sprees. Barring a fundamental shift in public opinion, Obama said, "it will not change." (AP Photo/Jacquelyn Martin)
President Barack Obama pauses while speaking during a memorial ceremony, Wednesday, April 9, 2014, at Fort Hood, Texas, for those killed there in a shooting last week. President Barack Obama is reprising his role as chief comforter as he returns once again to a grief-stricken corner of America to mourn with the families of those killed last week at Fort Hood and offer solace to the nation.(AP Photo/Carolyn Kaster)
President Barack Obama and first lady Michelle Obama arrive for a memorial ceremony, Wednesday, April 9, 2014, at Fort Hood Texas, for those killed there in a shooting last week. President Barack Obama is reprising his role as chief comforter as he returns once again to a grief-stricken corner of America to mourn with the families of those killed last week at Fort Hood and offer solace to the nation.(AP Photo/Carolyn Kaster)
President Barack Obama is seen on stage before speaking about yesterday's shooting at Fort Hood, during an event welcoming members of the USs teams from the 2014 Winter Olympic and Paralympic Winter Games in Sochi, Russia, Thursday, April 3, 2014, in the East Room of White House in Washington. (AP Photo/Pablo Martinez Monsivais)
AP10ThingsToSee - President Barack Obama hugs a family member of a victim of the Washington Navy Yard shooting at Marine Barracks in Washington during a memorial service Sunday, Sept. 22, 2013. A gunman killed 12 people in the Navy Yard on Sept. 16 before being fatally shot in a gun battle with law enforcement. (AP Photo/Charles Dharapak, File)
President Barack Obama and first lady Michelle Obama attend a memorial service for the victims of the Washington Navy Yard shooting at Marine Barracks Washington Sunday, Sept. 22, 2013. A gunman killed 12 people in the Navy Yard on Monday, Sept. 16, 2013, before being fatally shot in a gun battle with law enforcement. The president and first lady Michelle Obama also visited with the victims' families. (AP Photo/Charles Dharapak)
President Barack Obama pauses as he speaks in the South Court Auditorium on the White House complex, Monday, Sept. 16, 2013, in Washington. Before speaking about the economy Obama spoke on the shooting at the Washington Navy Yard, mourning what he called "yet another mass shooting" in the United States that he says took the life of American patriots. (AP Photo/Pablo Martinez Monsivais)
FILE - In this April 17, 2013 file photo President Barack Obama speaks in the White House Rose Garden of the White House about measures to reduce gun violence with former Rep. Gabby Giffords and family of victims of the Sandy Hook elementary school shooting. Six months after a gunman took their children's lives, some family members are headed back to Capitol Hill this week to remind lawmakers they are painfully waiting for action. From left are Neil Heslin, who lost his son Jesse Lewis; Giffords; Jimmy Greene, who lost his daughter Ana; Vice President Joe Biden; Nicole Hockley, who lost her son Dylan; Mark and Jackie Barden, with their children Natalie and James, who lost their son Daniel; and Jeremy Richman, behind the Barden's, who lost his daughter Avielle. (AP Photo/Carolyn Kaster, File)
President Barack Obama looks at Nicole Hockley and her husband Ian, right, after she introduced him at the University of Hartford in Hartford, Conn., Monday, April 8, 2013. The Hockley's lost a child in the school shooting at Sandy Hook Elementary School in Newtoen, Conn. Obama said that lawmakers have an obligation to the children killed and other victims of gun violence to act on his proposals. (AP Photo/Susan Walsh)
President Obama embraces a woman while honoring shooting victim's families during a visit to the University of Hartford, in Hartford, Conn., Monday, April 8, 2013. The President visited the school to highlight gun control legislation and to meet with the families of victims from the Sandy Hook elementary school shootings.(AP Photo/Charles Krupa)
FILE - In this Jan. 16, 2013, file photo, President Barack Obama, accompanied by Vice President Joe Biden, talks about proposals to reduce gun violence at the White House in Washington. Obama has called for a ban on military-style assault weapons and high-capacity ammunition magazines and is pushing other policies in the wake of the mass shooting last month at an elementary school in Newtown, Conn. In response, gun-rights advocates have accused Obama and others of ignoring the Second Amendment rights of Americans. (AP Photo/Charles Dharapak)
President Barack Obama, accompanied by Vice President Joe Biden, left, hugs eight-year-old letter writer Grant Fritz during a news conference on proposals to reduce gun violence, Wednesday, Jan. 16, 2013, in the South Court Auditorium at the White House in Washington. Obama and Biden were joined by law enforcement officials, lawmakers and children who wrote the president about gun violence following the shooting at an elementary school in Newtown, Conn., last month. (AP Photo/Susan Walsh)
FILE - This Dec. 16, 2012 file photo shows President Barack Obama pausing during a speech at an interfaith vigil for the victims of the Sandy Hook Elementary School shooting, at Newtown High School in Newtown, Conn. The National Rifle Association, the nationâs largest gun lobby, suggested shielding children from gun violence by placing an armed police officer in every school by the time classes resume in January. (AP Photo/ Evan Vucci, File)
President Barack Obama wipes his eye as he talks about the Connecticut elementary school shooting, Friday, Dec. 14, 2012, in the White House briefing room in Washington. (AP Photo/Carolyn Kaster)
President Barack Obama pauses as he speaks about the Aurora, Colo., shooting at an campaign event at the Harborside Event Center in Ft. Myers, Fla., Friday, July 20, 2012. Obama, who was scheduled to spend the day campaigning in Florida, cancelled his campaign events to return to Washington to monitor the shooting. (AP Photo/Susan Walsh)
FILE - In this March 14, 2011, file photo President Barack Obama speaks at Kenmore Middle School in Arlington, Va. More than five months after Rep. Gabrielle Giffords was shot in the head, the White House has yet to take any new steps on gun violence, even though thatâs what Obama called for in the wake of the shooting. (AP Photo, File)
President Barack Obama and first lady Michelle Obama are joined by government employees on the South Lawn of the White House in Washington, Monday, Jan. 10, 2011, to observe a moment of silence for Rep. Gabrielle Giffords, D-Ariz., and the other victims of an assassination attempt against her. The shooting at a town hall-style event outside a supermarket in Tucson, Ariz., Saturday left six dead, including a federal judge, and critically wounded Rep. Gabrielle Giffords. (AP Photo/J. Scott Applewhite)
President Barack Obama pauses as he speaks at a memorial service for the victims of Saturday's shootings at McKale Center on the University of Arizona campus Wednesday, Jan. 12, 2011, in Tucson, Ariz. (AP Photo/J. Scott Applewhite)
Victims' family members, along with President Obama, attend a memorial service at Fort Hood, Texas, for the victims of the Fort Hood shootings on Tuesday Nov. 10, 2009. (AP Photo/Jay Janner, POOL)
President Barack Obama speaks at a memorial service in Fort Hood, Texas on Tuesday, Nov. 10, 2009 for the victims of Thursday's shootings. (AP Photo/Pool, Jay Janner)
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Though the moves probably wouldn't have prevented recent mass shootings, Obama rejected the idea that undermines the changes.
"We maybe can't save everybody, but we could save some," Obama said.
A look at how some recent mass shooting suspects got their weapons:
Dec. 2, 2015, San Bernardino, California, 14 killed
Syed Farook and his wife used weapons that the FBI says his neighbor, Enrique Marquez, purchased legally from a federally licensed dealer in 2011 and 2012. Marquez, now facing conspiracy and other charges, told investigators that Farook asked him to purchase the weapons because he would draw less attention. At the time, the FBI says, the men were plotting to shoot up a community college and a highway.
Oct. 1, 2015, Roseburg, Oregon, 10 killed
Christopher Harper-Mercer and his family members legally purchased the handguns and rifle he used in the Umpqua Community College shooting from a federally licensed gun dealer, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives.
July 16, 2015, Chattanooga, Tennessee, 5 killed
The FBI says some of the weapons Muhammad Youssef Abdulazeez used in his attack on a pair of military facilities were purchased legally and some were not. It is unclear when the purchases were made and whether he was subject to a background check. Relatives say Abdulazeez had a history of mental illness, made a series of overseas trips and was arrested in April on suspicion of driving under the influence of alcohol. In May 2013, he failed a background check for an engineering job at a nuclear power plant in Ohio.
June 17, 2015, Charleston, South Carolina, 9 killed
A February drug arrest should have prevented Dylann Roof from purchasing the pistol authorities say he used at Emanuel AME Church, but a record-keeping error and background check delay allowed the transaction to go through. The FBI says a background check examiner never saw the arrest report because the wrong arresting agency was listed in state criminal history records. After three days passed, the gun dealer was legally permitted to complete the transaction.
Sept. 16, 2013, Washington, D.C., 12 killed
Aaron Alexis, a former reservist turned civilian contractor, passed state and federal background checks and legally purchased the pump-action shotgun used in the Washington Navy Yard shooting despite a history of violent outbursts and recent mental health treatment. Alexis was accused of firing a gun in anger in Texas in 2004 and Seattle in 2010, but was not prosecuted in either case. In 2011, he received an honorable discharge despite bouts of insubordination, disorderly conduct and unauthorized absences. None of that would have disqualified him from purchasing a weapon.
Dec. 14, 2012, Newtown, Connecticut, 26 killed
Adam Lanza used his mother's weapons, including a .223-caliber semi-automatic rifle, in the massacre at Sandy Hook Elementary School. Investigators say Lanza's mother, whom he fatally shot before going to the school, also purchased the ammunition.
July 20, 2012, Aurora, Colorado, 12 killed
James Holmes was receiving psychiatric treatment when he passed required federal background checks and legally purchased the weapons he used in the movie theater assault. As in the Navy Yard case, Holmes' treatment alone would not have disqualified his purchases. They would have been blocked if had he been adjudicated as a mental defective or committed to a mental institution.
Look back at the deadliest mass shootings in U.S. history:
Deadliest Mass Shootings in U.S. History
Obama measures wouldn't have kept guns from mass shooters
TOPSHOT - Mourners hold up signs during a vigil in Washington, DC on June 12, 2016, in reaction to the mass shooting at a gay nightclub in Orlando, Florida.
Fifty people died when a gunman allegedly inspired by the Islamic State group opened fire inside a gay nightclub in Florida, in the worst terror attack on US soil since September 11, 2001. / AFP / Andrew Caballero-Reynolds (Photo credit should read ANDREW CABALLERO-REYNOLDS/AFP/Getty Images)
ORLANDO, FL - JUNE 12: FBI agents investigate near the damaged rear wall of the Pulse Nightclub where Omar Mateen allegedly killed at least 50 people on June 12, 2016 in Orlando, Florida. The mass shooting killed at least 50 people and injuring 53 others in what is the deadliest mass shooting in the countryÃs history. (Photo by Joe Raedle/Getty Images)
WEST HOLLYWOOD, CA - JUNE 12: A guest holds a sign remembering the Orlando Massacre at the LA PRIDE Music Festival and Parade 2016 on June 10, 2016 in West Hollywood, California. (Photo by Joe Scarnici/WireImage)
Virginia Tech, 32 killed
Blacksburg police officers run from Norris Hall on the Virginia Tech campus in Blacksburg, Va., Monday, April 16, 2007. Multiple shootings occurred at the engineering building on Monday. A gunman opened fire in a Virginia Tech dorm and then, two hours later, in a classroom across campus Monday, killing at least 30 people. (AP Photo/The Roanoke Times, Matt Gentry)
Susan Hylton, left, hugs her daughter, Mary McFillin, both of Vienna, Va., Monday, April 16, 2007, after Hylton arrived on campus to pick up her daughter. McFillin, 19, is a student at Virginia Tech. A gunman killed 32 people at Virginia Tech Monday in the deadliest shooting rampage in modern U.S. history. (AP Photo/Richmond Times-Dispatch, Dean Hoffmeyer)
BLACKSBURG, VA - APRIL 17: Thousands of people attend a candlelight vigil on the campus of Virginia Tech April 17, 2007 in Blacksburg, Virginia. According to police, English major Cho Seung-Hui, 23, a native of South Korea, went on a shooting rampage that left a total of 33 people dead. It was the worst mass shooting in U.S. history. (Photo by Chip Somodevilla/Getty Images)
Sandy Hook Elementary, 27 killed
Carlee Soto uses a phone to get information about her sister, Victoria Soto, a teacher at the Sandy Hook elementary school in Newtown, Conn. Friday, Dec. 14, 2012 after a gunman killed over two dozen people, including 20 children. Victoria Soto, 27, was among those killed. (AP Photo/Jessica Hill)
Emergency vehicles line the road at a firehouse staging area for family at the entrance to Sandy Hook School, the site of a school shooting in Newtown, Conn., Friday, Dec. 14, 2012. A man opened fire Friday inside two classrooms at the school where his mother worked as a teacher, killing 26 people, including 20 children. The killer, armed with two handguns, committed suicide at the school and another person was found dead at a second scene, bringing the toll to 28, authorities said. A law enforcement official identified the gunman as 20-year-old Adam Lanza. (AP Photo/Jessica Hill)
Parents leave a staging area after being reunited with their children following a shooting at the Sandy Hook Elementary School in Newtown, Conn., about 60 miles (96 kilometers) northeast of New York City, Friday, Dec. 14, 2012. An official with knowledge of Friday's shooting said 27 people were dead, including 18 children. It was the worst school shooting in the country's history. (AP Photo/Jessica Hill)
Luby's Cafeteria, 23 killed
Police officers gather outside Luby's Cafeteria in Killeen, Texas, at the scene where a gunman killed 23 people including himself, with semi-automatic gunfire during lunchtime on Wednesday, Oct. 16, 1991. It was the deadliest mass shooting in U.S. history. (AP Photo/Rick McFarland)
Unidentified mourners comfort each other after a funeral service for Michael Griffith at Grace United Methodist Church in Copperas Cove, Texas on Sunday, Oct. 20, 1991. Griffith was among the people who were killed on Wednesday in the massacre at the Luby's Cafeteria in Killeen, Texas. (AP Photo/Eric Gay)
Officials remove bodies from Luby's Cafeteria off Interstate I-90 at Killeen, Bell County, Texas, on Wednesday night, Oct. 16, 1991. The gunman, George Hennard, crashed his pickup truck through the window of the restaurant at lunchtime and opened fire, killing 23 people and wounding 20 others, before killing himself. (AP Photo/Ron Heflin)
McDonald's in San Ysidro, Calif., 21 killed
A San Diego police officer helps an injured woman away from the scene of a shooting at a McDonald's restaurant in San Ysidro, Calif., July 18, 1984. The woman had been in the restaurant when a gunman opened fire with an automatic weapon killing 20 people and wounding another dozen. Police said the woman had family with her in the restaurant. (AP Photo/Lenny Ignelzi)
This view is from the northeast of the San Ysidro McDonald's in San Diego, shown July 19, 1984. West San Ysidro Blvd. is in foreground. Most of the bodies found outside the restaurant were along this side, in front of the parked cars alongside the store. (AP Photo/Vince Bucci)
Gloria Aquino, right, shares her sorrow with a friend during burial services for her sister, Paulina Aquino, who was gunned down in the McDonald's massacre in San Ysidro, California last Wednesday. Services were on July 21, 1984 in Tijuana, Mexico (AP Photo/Jimmy Dorantes)
University of Texas at Austin, 18 killed
Ambulance attendants remove the body of Mrs. Kathleen Whitman, wife of Charles Joseph Whitman, from the family home in Austin on August 1, 1966. Whitman was identified by police as the slayer of 16 persons including his wife and mother during a wild shooting spree. (AP Photo/DO)
FILE - This Aug. 1, 1966 file photos shows the weapons used by Charles Joseph Whitman in his mad shooting spree Aug. 1, 1966 in which 16 persons were killed and a score more wounded, in Austin, Texas. Police seized the weapons after they gunned down Whitman in his perch in the University of Texas administration building tower. A rifle used in the 1966 sniper rampage is being sold in an online auction. Bids are starting at US$ 25,000. (AP PHOTO/FILES)
FILE - In this Aug. 2, 1966 file photo, from left, Allen Crum, university co-op employee; Austin policemen Ramiro Martinez, Houston McCoy and Jerry Day, the four men who braved the deadly accurate sniper fire by Charles Joseph Whitman from the University of Texas tower, meet with newsmen in Austin, Texas. McCoy, the Austin police officer who fired two blasts from his shotgun to bring down Whitman, has died. (AP Photo/Ted Powers, File)
U.S. Post Office in Edmond, Okla., 14 killed
An Edmond, Okla., postal worker is comforted after the Aug. 20, 1986, shooting spree by Patrick Sherrill that killed 14 of his co-workers. Until the Oklahoma City bombing, it was the deadliest one-day attack in state history. (AP Photo/David Longstreath)
Medical personnel rush a wounded man to a waiting ambulance in this Aug. 20, 1986 file photo, following a shooting spree at U.S. Post Office in Edmond, Okla., where U.S. Postal employee Patrick Henry Sherrill shot and killed 14 fellow employees before taking his own life. Over the past 20 years, the Post Office has tried to make improvements to prevent such acts of violence. (AP Photo/David Longstreath)
Police stand near shooting spree victims at the Edmond Post Office, Edmond, Oklahoma Wednesday, August 20, 1986. A postal employee shot and killed 14 people before taking his own life. (AP Photo/Steve Gooch)
San Bernardino holiday party, 14 killed
This July 27, 2014 photo provided by U.S. Customs and Border Protection shows Tashfeen Malik, left, and Syed Farook, as they passed through O'Hare International Airport in Chicago. The husband and wife died on Dec. 2, 2015, in a gun battle with authorities several hours after their assault on a gathering of Farook's colleagues in San Bernardino, Calif. (U.S. Customs and Border Protection via AP)
An investigator looks at a Black SUV that was involved in a police shootout with suspects, Thursday, Dec. 3, 2015, in San Bernardino, Calif. A heavily armed man and woman opened fire Wednesday on a holiday banquet, killing multiple people and seriously wounding others in a precision assault, authorities said. Hours later, they died in a shootout with police. (AP Photo/Jae C. Hong)
Trung Do Nguyen, right, comforts his mother, Van Thanh Nguyen, at a wake for his sister and her daughter, Tin Nguyen, at the Peek Funeral home in Westminster, Calif., on Friday, Dec. 11, 2015. Nguyen died in the mass shootings in San Bernardino, Calif., last Wednesday, Dec. 2. Woman at left is unidentified.(AP Photo/Nick Ut)
Columbine High School, 13 killed
A woman embraces her daughter after they were reunited following a shooting at Columbine High School in Littleton, Colo., on Tuesday, April, 20, 1999. As many as 25 were killed at the school by two young men who went on a shooting rampage on Tuesday. (AP Photo/Ed Andrieski)
LITTLETON, CO - APRIL 20: (VIDEO CAPTURE) Columbine high school shooters Eric Harris (L) and Dylan Klebold appear in this video capture of a surveillance tape released by the Jefferson County Sheriff's Department in the cafeteria at Columbine High School April 20, 1999 in Littleton, CO during their shooting spree which killed 13 people. (Photo courtesy of Jefferson County Sheriff's Department via Getty Images)
Jefferson Country Sheriff's Deputy Aaron Rollins, left, gives Duchess of York, Sarah Ferguson, a guided tour of the makeshift memorial near Columbine High School in Littleton, Colo., on Tuesday, April 27, 1999. Two students went on a shooting rampage at the school last week, killing 12 other students and a teacher before killing themselves. (AP Photo/Michael S. Green)
Fort Hood, Texas, 13 killed
KILLEEN, TX - NOVEMBER 5: Sgt. Fanuaee Vea embraces Pvt. Savannah Green while trying to reach friends and family outside Fort Hood on November 5, 2009 in Killeen, Texas. At least one gunman killed 12 people and injured 31 in a shooting on a military base at Fort Hood this afternoon. One shooter was killed by military police and at least two other soldiers are in custody. (Photo by Ben Sklar/Getty Images)
U.S. Army Lt. Gen. Bob Cone, right, speaks during a news conference outside Fort Hood, Texas, Thursday, Nov. 5, 2009. Cone announced that a mass shooting suspect is in custody and not dead. The suspect is in stable condition. (AP Photo/LM Otero)
Sgt. Anthony Sills, right, comforts his wife as they wait outside the Fort Hood Army Base near Killeen, Texas on Thursday, Nov. 5, 2009. The Sills' 3-year old son is still in daycare on the base, which is in lock-down following a mass shooting earlier in the day. (AP Photo/Jack Plunkett)
Binghamton, N.Y., 13 killed
Law enforcement personnel investigate outside the American Civic Association, Friday, April 3, 2009, in Binghamton, N.Y. A gunman opened fire on a center where immigrants were taking a citizenship exam Friday in downtown Binghamton, killing 13 people before apparently committing suicide. (AP Photo/Heather Ainsworth)
FILE - Police officers help hostages exit a building near the American Civic Association in downtown Binghamton, N.Y., following a shooting spree by a gunman in this April 3, 2009 file photo. (AP Photo/Press & Sun-Bulletin, Rebecca Catlett)
Unidentified people embrace outside a Catholic Charities office where counselors tend to relatives of victims of the shooting in Binghamton N.Y., Friday, April 3, 2009. A gunman barricaded the back door of a community center with his car and then opened fire on a room full of immigrants taking a citizenship class Friday, killing 13 people before apparently committing suicide, officials said. (AP Photo/Matt Rourke)
Wah Mee gambling club, 13 killed
Wai Chiu "Tony" Ng looks on Wednesday, Feb. 14, 2007, prior to the official start of a hearing before the Indeterminate Sentence Review Board at the McNeil Island Corrections Center on McNeil Island, Wash. Ng was acquitted of murder and convicted of 13 counts of first-degree robbery and one count of second-degree assault for his role in the Feb. 19, 1983, killings of 13 people at the Wah Mee gambling club in Seattle. He was sentenced to serve seven consecutive life terms, and the ISRB has paroled him on the first five counts. Wednesday's hearing was to consider the sixth count. (AP Photo/Ted S. Warren, Pool)
Wai Chiu "Tony" Ng, center, facing the review board, and his attorney, George Marlton, to his right, and his Department of Corrections counselor Donald Walston, left, face Julia Garratt, second from left, and Dennis Thaut, both of the Indeterminate Sentence Review Board, Wednesday, Feb. 14, 2007, prior to the start of an ISRB parole hearing on McNeil Island, Wash. Ng was acquitted of murder and convicted of 13 counts of first-degree robbery and one count of second-degree assault for his role in the Feb. 19, 1983, killings of 13 people at the Wah Mee gambling club in Seattle. He was sentenced to serve seven consecutive life terms, and the ISRB has paroled him on the first five counts. Wednesday's hearing was to consider the sixth count. (AP Photo/Ted S. Warren, Pool)
McNeil Island Correctional Center is shown in this view from the water Wednesday, Feb. 14, 2007. Wai Chiu "Tony" Ng, who was acquitted of murder and convicted of 13 counts of first-degree robbery and one count of second-degree assault for his role in the Feb. 19, 1983, killings of 13 people at the Wah Mee gambling club in Seattle is being held here. (AP Photo/Ted S. Warren, Pool)
Aurora, Colo. movie theater, 12 killed
Police are pictured outside of a Century 16 movie theatre where as many as 14 people were killed and many injured at a shooting during the showing of a movie at the in Aurora, Colo., Friday, July 20, 2012. (AP Photo/Ed Andrieski)
Tom Sullivan, center, embraces family members outside Gateway High School where he has been searching franticly for his son Alex Sullivan who celebrated his 27th birthday by going to see "The Dark Knight Rises," movie where a gunman opened fire Friday, July 20, 2012, in Aurora, Colo. (AP Photo/Barry Gutierrez)
A man walks on a hill near crosses set up at the memorial to victims of the Aurora, Colo., movie theater shooting, early Friday, July 27, 2012. It was a week ago Friday that a gunman opened fire during a late-night showing of "The Dark Knight Rises" Batman movie, killing 12 and injuring dozens of others. Police have identified the suspected shooter as James Holmes, 24. (AP Photo/Ted S. Warren)
Washington Navy Yard, 12 killed
People hold their hands to their heads as they are escorted out of the building where a deadly shooting rampage occurred at the Washington Navy Yard in Washington, Monday, Sept. 16, 2013. One shooter was killed, but police said they were looking for two other possible gunmen wearing military-style uniforms. (AP Photo/Jacquelyn Martin)
Emergency personnel respond to a reported shooting at the Washington Navy Yard, Monday, Sept. 16, 2013 in Washington. Police and federal agents from multiple law enforcement agencies responded to the scene. Ambulances were parked outside, streets in the area were closed and departures from Reagan National Airport were temporarily halted for security reasons. (AP Photo/Manuel Balce Ceneta)
Police who responded to shooting at the Washington Navy Yard Monday, Sept. 16, 2013, leave the facility. At least one gunman launched an attack inside the Washington Navy Yard, spraying gunfire on office workers in the cafeteria and in the hallways at the heavily secured military installation in the heart of the nation's capital, authorities said. (AP Photo/Manuel Balce Ceneta)