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.

14 PHOTOS Recent shootings by police and related protests See Gallery More US police charged with murder, manslaughter in 2015 People take part in a protest against police brutality and in support of Black Lives Matter during a march in New York, U.S. July 9, 2016. REUTERS/Eduardo Munoz/File Photo Lastarla Barker protests with other demonstrators calling for the Oakland Police Department to be defunded and against police brutality in Oakland, California, U.S. July 21, 2016. REUTERS/Noah Berger PORTLAND, USA - APRIL 11: A protester holds a sign reading 'End White Supremacy And The Police State' during a protest against fatal police shootings in Portland, Oregon, United States on April 11, 2018. John Andrew Elifritz, 48, was fatally shot by police after he reportedly fled from a stolen car and burst into a homeless shelter at the start of an alcoholics anonymous meeting last Saturday. (Photo by Alex Milan Tracy/Anadolu Agency/Getty Images) CHICAGO, USA - APRIL 2 : People attend a protest near Trump International Hotel and Tower in response to the police shootings of unarmed victims across the US on April 2, 2018 in Chicago, United States. (Photo by Bilgin Sasmaz/Anadolu Agency/Getty Images) People march in protest to the fatal police shooting of Charleena Lyles, in Seattle, Washington on June 22, 2017. Police in Washington were under scrutiny after a pregnant woman was fatally shot by officers responding to a burglary call. Authorities said the 30-year-old victim, identified as Charleena Lyles, had called to report an attempted burglary at her apartment on the morning of June 18 and pulled a knife on the two officers, who shot and killed her. / AFP PHOTO / Jason Redmond (Photo credit should read JASON REDMOND/AFP/Getty Images) SEATTLE, WA - JUNE 20: Chalk artwork is written on the ground at a memorial for Charleena Lyles at the apartment building in which she was killed on June 20, 2017 in Seattle, Washington. Officers from the Seattle Police Department shot and killed Lyles, a pregnant mother of four, on June 18. (Photo by David Ryder/Getty Images) The car of Philando Castile is seen surrounded by police vehicles in an evidence photo taken after he was fatally shot by St. Anthony Police Department officer Jeronimo Yanez during a traffic stop in July 2016. Picture released June 20, 2017. Minnesota Bureau of Criminal Apprehension/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. THIS PICTURE WAS PROCESSED BY REUTERS TO ENHANCE QUALITY. AN UNPROCESSED VERSION WILL BE PROVIDED SEPARATELY. People hold signs in protest after a jury found St. Anthony Police Department officer Jeronimo Yanez not guilty of second-degree manslaughter in the death of Philando Castile yesterday, in Manhattan, New York, U.S., June 17, 2017. REUTERS/Bria Webb 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 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) Portland, United States - May 19: Protesters hold a banner during a demonstration for freedom and equality against police brutality and racism at the Portland Police Bureau's North Precinct in Portland, Ore., United States, on May 19, 2017, on what would have been Malcolm X's 92nd birthday. (Photo by Alex Milan Tracy/Anadolu Agency/Getty Images) Portland, United States - May 19: Protesters hold signs during a demonstration for freedom and equality against police brutality and racism at the Portland Police Bureau's North Precinct in Portland, Ore., United States, on May 19, 2017, on what would have been Malcolm X's 92nd birthday. (Photo by Alex Milan Tracy/Anadolu Agency/Getty Images) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

"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.