CHICAGO (AP) -- The number of U.S. police officers charged with murder or manslaughter for on-duty shootings has tripled this year -- a sharp increase that at least one expert says could be the result of more video evidence.
In the past, the annual average was fewer than five officers charged. In the final weeks of 2015, that number has climbed to 15, with 10 of the cases involving video.
"If you take the cases with the video away, you are left with what we would expect to see over the past 10 years -- about five cases," said Philip Stinson, the Bowling Green State University criminologist who compiled the statistics from across the nation. "You have to wonder if there would have been charges if there wasn't video evidence."
The importance of video was highlighted last week with the release of footage showing a Chicago officer fatally shooting a teenager 16 times. The officer said he feared for his life from the teen, who was suspected of damaging cars using a small knife. He also had a powerful hallucinogen in his bloodstream.
Shootings by police, police brutality
More US police charged with murder, manslaughter in 2015
Protesters hold placards against the killing of Alton Sterling and Philando Castile in Manhattan, New York, U.S., July 7, 2016. REUTERS/Bria Webb
People take part in a protest against the killings of Alton Sterling and Philando Castile during a march in New York July 7, 2016. REUTERS/Eduardo Munoz
FILE - In this Tuesday, Nov. 24, 2015, file photo, a protester holds a sign as people rally for 17-year-old Laquan McDonald, who was shot 16 times by Chicago Police Department Officer Jason Van Dyke in Chicago. McDonald, whose name demonstrators are shouting as they march the streets and plan to shut down the cityâs glitziest shopping corridor on Friday, lived a troubled life full of disadvantages and at least one previous brush with the law. (AP Photo/Paul Beaty, File)
In this frame grab image made from an Oct. 12, 2014 video released by Chicago Police Department, Ronald Johnson, right, is seen running from police officers just a second before he was shot by an officer. Prosecutors say a Chicago police officer will not be charged in the shooting of the 25-year-old black man who authorities said was armed with a gun as he ran away from officers. (Chicago Police Department via AP)
Matthew White protests the shooting death of Michael Brown by police nearly a week ago Friday, Aug. 15, 2014, in Ferguson, Mo. A suburban St. Louis police chief on Friday identified the officer whose fatal shooting ignited days of heated protests, and released documents alleging the teen was killed after a robbery in which he was suspected of stealing a box of cigars. (AP Photo/Charlie Riedel)
CORRECTS THE ID OF THE MALE ON POSTER TO TAMIR RICE - Tomiko Shine holds up a picture of Tamir Rice, the 12 year old boy fatally shot on Nov. 22 by a rookie police officer, during a protest in response to a grand jury's decision in Ferguson, Mo. to not indict police officer Darren Wilson in the shooting death of Michael Brown, an unarmed black man, at the Department of Justice in Washington, Monday, Dec. 1, 2014. Protesters across the U.S. have walked off their jobs or away from classes in support of the Ferguson protesters. Rice's death has also sparked community demonstrations against police shootings. (AP Photo/Jose Luis Magana)
FILE - In this July 19, 2015 file photo from a body camera video provided by the University of Cincinnati Campus Police, university Officer Ray Tensing stands next to motorist Samuel DuBose during a traffic stop for a missing front license plate in Cincinnati. DuBose was fatally shot by the officer after a struggle ensued when he refused to provide a driver's license and get out of the car. Tensing was indicted Wednesday, July 29 on a murder charge. (University of Cincinnati Campus Police via AP, File)
Muhiydin D'Baha leads a group protesting the shooting death of Walter Scott at city hall in North Charleston, S.C., Wednesday, April 8, 2015. Scott was killed by a North Charleston police office after a traffic stop on Saturday. The officer, Michael Thomas Slager, has been charged with murder. (AP Photo/Chuck Burton)
A protestor carries a casket-shaped sign that reads "Justice for Antonio Zambrano Montes" as she takes part in a protest against recent shootings of unarmed civilians by police Tuesday, April 14, 2015, in Seattle. Zambrano-Montes was shot Feb. 10, 2015, by police in Pasco, Wash. (AP Photo/Ted S. Warren)
FILE - In this Dec. 4, 2014 file photo, demonstrators participate in a rally against a grand jury's decision not to indict the police officer involved in the death of Eric Garner, in New York. In the days since grand juries in Missouri and New York decided against indicting white police officers in the deaths of black men, protesters nationwide have demanded a reckoning and an acknowledgement that âblack lives matter.â Yet so far, there are few signs such a conversation will come in the place where it might most make a difference: the next campaign for president. (AP Photo/John Minchillo, File)
FILE - This undated photo released by his sister Javille Burns shows Jamar Clark. Clark was involved in a Nov. 15 confrontation with police and died later. Officers said Clark was shot after a struggle. Others say Clark was handcuffed. His death sparked weeks of protests.(Jamar Clark/Javille Burns via AP, File)
NEW YORK, NY - DECEMBER 06: Demonstrators march through the streets protesting the Staten Island, New York grand jury's decision not to indict a police officer involved in the chokehold death of Eric Garner in July on December 6, 2014 in New York City. Protests are being staged nationwide after grand juries investigating the deaths of Michael Brown in Ferguson, Missouri and Eric Garner in New York failed to indict the police officers involved in both incidents. (Photo by Andrew Burton/Getty Images)
A protest sign showing and image of Ezell Ford as members of the 'Black Lives Matter' alliance stage protest outside the Los Angeles Mayor Eric Garcetti's home as they try to force him to fire LAPD Police Chief Charlie Beck, in Los Angeles, California on June 7, 2015. The alliance have renewed protests after a recent report from an LAPD watchdog determined that the August 11, 2014 officer-involved shooting death of 25-year-old Ezell Ford in South Central was justified. AFP PHOTO/ MARK RALSTON (Photo credit should read MARK RALSTON/AFP/Getty Images)
Protestors stand outside of the Baltimore Police Department's Western District police station during a march and vigil for Freddie Gray, Tuesday, April 21, 2015, in Baltimore. Gray died from spinal injuries a week after he was arrested and transported in a police van. (AP Photo/Patrick Semansky)
In this undated photo provided by the Philadelphia District Attorney's Office, Najee Rivera is shown. Philadelphia Police Office Sean McKnight and Kevin Robinson face brutality charges after prosecutors say they knocked a Rivera off a scooter and beat him so severely another officer thought the bloodied man had been shot. McKnight and Robinson were charged Thursday, Feb. 5, 2015 with assault, criminal conspiracy and reckless endangerment. They're also charged with lying about the May 2013 incident. (AP Photo/Philadelphia District Attorney's Office)
Two men are detained near Pioneer Court on Friday, Nov. 27, 2015, in Chicago. Community activists and labor leaders held a demonstration billed as a "march for justice" in the wake of the release of video showing an officer fatally shooting Laquan McDonald. (AP Photo/Nam Y. Huh)
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"This had all the trappings of a life-threatening situation for a law-enforcement officer -- PCP-laced juvenile who had been wreaking havoc on cars with a knife," said Joseph Tacopina, a prominent New York defense attorney and former prosecutor who has represented several police officers. "Except you have the video that shows a straight-out execution."
When he was charged with first-degree murder last week, officer Jason Van Dyke became the 15th officer in the country to face such charges in 2015.
Over the last decade, law-enforcement agencies have recorded roughly 1,000 fatal shootings by on-duty police. An average of fewer than five each year resulted in murder or manslaughter charges against officers, Stinson found.
The cases are often difficult to prove. Of the 47 officers charged from the beginning of 2005 through the end of last year, about 23 percent were convicted, Stinson found.
"For forever, police have owned the narrative of what happened between any encounter between a police officer and a civilian," said David A. Harris, a University of Pittsburgh law professor who has written extensively on police misconduct. "What video does is it takes that power of the narrative away from the police to some extent. And that shift in power of control over the narrative is incredibly significant."
In case after case, that is exactly what has happened this year.
Stinson said Van Dyke would "never, ever" have been charged without the video. He said the same is true for Ray Tensing, the white University of Cincinnati police officer who is charged with murder and involuntary manslaughter in the July 19 death of Samuel DuBose, a black motorist whom Tensing shot to death after pulling him over for a missing front license plate.
Tensing's attorney said the officer feared he would be dragged under the car as Dubose tried to drive away. But, Stinson said, the video from the officer's body camera shows that his explanation "doesn't add up."
Other cases around the country also reveal just how important the video is.
In Marksville, Louisiana, for example, two deputy city marshals were charged with second-degree murder after authorities reviewed video from one of the officers' body cameras, which showed a man with his hands in the air inside a vehicle when the marshals opened fire. The man was severely wounded and his 6-year-old autistic son killed.
Just how dramatically a video can shift the balance of power was apparent in North Charleston, South Carolina, when officer Michael Slager shot and killed Walter Scott, an unarmed black man as he ran away after a traffic stop.
Slager told investigators that Scott had tried to grab his gun and Taser. But after a video from a cellphone showed Slager taking careful aim at Scott as he ran away and then picking up his Taser and dropping it near Scott's body, Slager was charged with murder.
"If not for the recording, I have no doubt that the officer in the Walter Scott case would be out on patrol today," Harris said.
Videos have also played a key role in cases in which the victims were, in fact, armed -- something that Tacopina said typically brings to a halt any thought of charging officers.
Chicago prosecutors concluded that McDonald did not pose a threat to Van Dyke, despite the small knife that he was carrying.
Likewise, prosecutors in Albuquerque, New Mexico, charged two officers with second-degree murder of a mentally ill homeless man who was holding two knives when he was shot to death. Defense attorneys have said the officers shot James Boyd out of concern for their lives, but Boyd appears to be turning away from the officers when the shots were fired.
In another case, an officer may owe her freedom to the camera that was attached to her stun gun.
Lisa Mearkle, a police officer in Hummelstown, Pennsylvania, was charged with third-degree murder, voluntary and involuntary manslaughter after shooting an unarmed man twice in the back as he laid face-down in the snow. But after watching a video that showed the man's hands repeatedly disappear under his body as Mearkle shouted at him to keep his hands where she could see them, the jury acquitted Mearkle.