The U.S. Supreme Court suspended the death penalty in 1972, but it was reinstated just four years later after the 1976 Gregg v. Georgia. Several historic cases have shaped the active conversation on capital punishment -- here is a look at four of them.
Cameron Todd Willingham was convicted of murdering his three daughters by allegedly setting his house on fire in 1991. Willingham maintained his innocence at his trial despite the fact that a guilty plea would spare him from the death penalty.
New techniques for investigating fires were developed during the time Willingham sat on death row. A fire inspector reviewed the case and reported that he didn't believe the evidence pointed fairly to arson. Despite this, Willingham was executed in February 2004.
In 2009, a formal review of the case by the Texas Forensic Science Commission concluded that the evidence used in the case was merely personal opinion not based on scientific evidence and the fire marshal's declaration of arson was unfounded.
See more on these controversial cases:
Controversial Death Penalty Cases
Controversial death penalty cases
Dave Atwood, left, and Sophia Malik, right, both of Houston, hold photos of Napoleon Beazley as they protest his execution Tuesday, May 28, 2002, in Huntsville, Texas. Beazley, 25, was executed by lethal injection for the 1994 carjacking murder of 63-year-old John E. Luttig of Tyler, Texas. It was the 14th execution this year in Texas. (AP Photo/Brett Coomer)
Rena, left, and Ireland Beazley hold a photo of their son Napoleon Beazley at their home in Grapeland, Texas, Friday, May 31, 2002. Napoleon Beazley's death sentence for killing the father of a federal judge during a 1994 carjacking at age 17 stirred national debate over capital punishment for youths. (AP Photo/Donna McWilliam)
Rena Beazley, left, and her husband, Ireland, from Grapeland, Texas, are shown in the audience during a news conference Thursday, May 23, 2002, in Austin, Texas. The two, parents of Texas death row inmate Napoleon Beazley, and clergy pleaded for his sentence to be commuted to life in prison. He is scheduled to be executed by lethal injection Tuesday. (AP Photo/Harry Cabluck)
Mugshot of Cameron Todd Willingham
(Photo credit: Texas Department of Criminal Justice)
Judy Cavnar, of Ardmore, Okla., a cousin of executed Texas prison inmate Cameron Todd Willingham, displays a picture of him during a news conference Tuesday, May 2, 2006, in Austin, Texas. The case of a Willingham, who maintained his innocence until the end but was executed after he was convicted of an arson murder, is going before a new state commission required to look into allegations of forensic misconduct. (AP Photo/Harry Cabluck)
Eugenia Willingham of Ardmore, Okla., right, wipes a tear as she speaks during a news conference Tuesday, May 2, 2006, in Austin, Texas. Willingham and other relatives of Cameron Todd Willingham recounted the final moments of Willingham's life and their unsuccessful attempts to block his execution. The New York-based Innocence Project submitted the case to the Texas Forensic Science Commission on Tuesday and also asked the panel to review arson convictions statewide. In the background, from left are Willingham's cousins, Pat Cox, and Judy Cavnar. Mrs. Willingham is his stepmother. (AP Photo/Harry Cabluck)
Death row inmate Troy Davis appears in this undated file photo provided by the Georgia Department of Corrections. (Georgia Department of Corrections/MCT via Getty Images)
Demonstrators gather in front of the White House in Washington as they hold a vigil before the scheduled execution of death row inmate Troy Davis, Wednesday, Sept. 21, 2011. Davis is facing lethal injection for killing an off-duty Georgia policeman in Savannah, a crime he and others have insisted for years that he did not commit. (AP Photo/Charles Dharapak)
A man chants during a vigil for Georgia death row inmate Troy Davis In Jackson, Ga., Wednesday, Sept. 21, 2011. Davis is scheduled to die Wednesday for the killing off-duty Savannah officer Mark MacPhail. (AP Photo/John Bazemore)
Anne MacPhail pauses for a moment after learning at 10:55 p.m., on September 21, 2011, that the U.S. Supreme Court had denied a stay of execution for Troy Davis, who was convicted in the 1989 murder of her son Mark MacPhail. Davis was executed shortly after in Jackson, Georgia. (Robin Trimarchi/Columbus Ledger-Enquirer/MCT via Getty Images)
Mugshot of Kelly Renee Gissendaner
(Photo credit: Georgia Department of Corrections)
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A football player and president of his high school class, Napoleon Beazley was sentenced to death for shooting a 63-year-old man in the head during a carjacking.
Beazley was 17 when he committed the crime. He had no prior criminal record. He also was too young to vote, too young to drink, and too young to serve in the military.
His capital murder trial began in January 1995. And in May 2002, having exhausted every possible appeal, Beazley was executed. He was 25 years old. According to The Espy File, 22 juveniles were executed between 1976 and 2005. In 2005, the Supreme Court ruled that the execution of juveniles is unconstitutional.
Troy Davis was convicted for the murder of off-duty Georgia police officer Mark MacPhail in 1989. Davis spent more than two decades on death row, maintaining his innocence the entire time.
Prosecutors found no murder weapon, the case against him included no physical evidence and consisted entirely of witness testimony -- testimony which contained inconsistencies even at the time of the trial. All but two of the state's non-police witnesses from the trial recanted or contradicted their testimony.
Pope Benedict XVI, Amnesty International, President Jimmy Carter and nearly a million other Davis supporters signed a petition to the Georgia Board of Pardons for clemency. Davis was executed by lethal injection on September 21st, 2011.
During his visit to the U.S., Pope Francis made a plea to save death row inmate Kelly Renee Gissendaner.
Gissendaner was sentenced to death for plotting the 1997 murder of her husband at the hands of her lover, who was sentenced to life in prison for the crime.
The victim's family was split on whether Gissendaner should live or die: her children asked the parole board to spare their mother, but her husband's relatives said she did not deserve clemency. Gissendanger became the fifth woman to be executed in the nation in the past decade.
Today, capital punishment is legal in 31 states. Japan is the only other industrial democracy where the death penalty is legal. The U.N continues to push for abolishment of the death penalty worldwide.
Click through for more notable executions and people on death row:
Notable death penalty executions and people on death row
Controversial death penalty cases
Willie Francis, the first known incident of failed execution by lethal injection, was executed on May 9, 1947. He was sentenced to death for the murder of his former boss, Andrew Thomas.
John Wayne Gacy, a rapist and serial killer responsible for the sexual assaults and murders of at least 33 men, was executed in Illinois on May 10, 1994.
Ted Bundy, a kidnapper, rapist, and serial killer responsible for the assaults and murders of dozens of young women, was executed in in Florida on January 24, 1989. His actual victim count remains unknown.
Aileen Wuornos, a serial killer responsible for the deaths of seven men, was executed in Florida on October 9, 2002.
(AP Photo/Peter Cosgrove)
Charles Starkweather, a spree killer responsible for eleven murders, was executed in Nebraska on June 25, 1959.
(AP Photo/Don Ultang)
Timothy McVeigh, responsble for the Oklahoma City bombing, was executed in Indiana on June 11, 2001.
Thomas Provenzano, a convicted murderer responsible for shooting three people, was executed in Florida on June 21, 2000.
(AP Photo/Peter Cosgrove)
Gary Gilmore, responsible for the shooting deaths of two men, was executed in Utah on January 17, 1977.
Stanley "Tookie" Williams, founder and leader of the Crips gang responsible for several murders and other crimes, was executed in California on December 13, 2005.
(Photo by Justin Sullivan/Getty Images)
Teresa Lewis, convicted of murdering her husband and stepson, was executed in Virginia on September 23, 2010.
(AP Photo/Va Dept of Corrections)
William Bonin, a serial killer responsible for a minimum of 21 rapes and murders, was executed in California on February 23, 1996.
Dzhokhar Tsarnaev, responsible for Boston Marathon bombing, was sentenced to death on May 15, 2015.
(AP Photo/Federal Bureau of Investigation, File)
Nidal Malik Hasan, responsible for Ford Hood shooting, was sentenced to death on August 28, 2013.
(AP Photo/Bell County Sheriff's Department, File)
Joseph E. Duncan III, a convicted murderer and sex offender, was sentenced to death on August 27, 2008.
(AP Photo/Kootenai County First Appearance Video Court)
Death row prisoner Coy Wayne Wesbrook is photographed Wednesday, Feb. 3, 2016, at the Texas Department of Criminal Justice Polunsky Unit outside Livingston, Texas. Wesbrook, 58, is set for lethal injection March 9, 2016, for the November 1997 fatal shootings of his ex-wife and another man at her apartment in Channelview, just east of Houston. They were among five people killed during the shooting rampage. (AP Photo/Michael Graczyk)