Body cameras, like this one shown at a 2014 press conference in Washington, D.C., are small enough to be clipped to an officer's chest. Washington and Denver are among U.S. cities trying the cameras. BRENDAN SMIALOWSKI/AFP/Getty Images hide caption

itoggle caption BRENDAN SMIALOWSKI/AFP/Getty Images

A Philadelphia police officer demonstrates a body-worn camera being used as part of a pilot project last December. Matt Rourke/AP hide caption

itoggle caption Matt Rourke/AP

Few police departments have required officers to wear body cameras, but that's changing after the events in Ferguson, Mo., this summer. Jim Mone/AP hide caption

itoggle caption Jim Mone/AP