Detective Mark Williams (right) speaks with an officer in Richmond, Va. A decade ago, amid a surge in violent crime, Richmond police were identifying relatively few murder suspects. So the police department refocused its efforts to bring up its "clearance rate." Alex Matzke for NPR hide caption

itoggle caption Alex Matzke for NPR

Some colleges and police departments are starting to use software that scans social media to identify local threats, but most tips still come from members of the public. Ikon Images/Getty Images hide caption

itoggle caption Ikon Images/Getty Images

Late last month, a scuffle cut short a St. Louis Board of Aldermen meeting where a committee was to discuss a proposed civilian review board for the city's police force. Robert Cohen/Courtesy of St. Louis Post-Dispatch hide caption

itoggle caption Robert Cohen/Courtesy of St. Louis Post-Dispatch

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