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

There's been a sharp decline in the number of arrests and tickets and summonses issued in New York City. Police sometimes use work slowdowns to show dissatisfaction with policies, workloads or contract disputes. Justin Lane/EPA/Landov hide caption

itoggle caption Justin Lane/EPA/Landov