The Buffalo, New York, department said a protester "tripped and fell." Philadelphia police alleged that a college student who suffered a serious head wound had assaulted an officer.

All three claims were quickly disproved by videos seen widely on the internet and television, fueling mistrust and embarrassing agencies that made misleading or incomplete statements that painted their actions in a far more favorable light.

Police departments deny lying but acknowledge sometimes making mistakes when releasing information in fast-moving, complicated situations. The videos, they say, do not always capture officers' perspectives.

Defense lawyers say the inaccurate statements are encouraged by a culture of silence in which officers protect misbehaving colleagues, a court system that rarely holds officers accountable and a public that has given police the benefit of the doubt.