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 partly 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 nine of the cases involving video.

David A. Harris is a University of Pittsburg law professor who has written extensively on police misconduct. He says video takes away from police the power to control the narrative of deadly confrontations.

The statistics were compiled by Philip Stinson, a Bowling Green State University criminologist. Of the 47 officers charged from 2004 through 2014, about 23 percent were convicted.