Metropolitan Division officers finish another "rollback" operation. They searched the apartment of a paroled armed robber and gang member. These rollbacks are a cornerstone of the Metro Division's strategy of tracking people who may re-offend, and suppressing crime before it happens. Martin Kaste/NPR hide caption

itoggle caption Martin Kaste/NPR

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

New Orleans police officer Patrick Schneider patrols Bourbon Street in the heart of the French Quarter on Saturday. Jason Saul/WWNO hide caption

itoggle caption Jason Saul/WWNO

Theodore Wafer (center) and his attorney Mack Carpenter sit in the back of the courtroom Jan. 15 before his arraignment in Detroit in the shooting death of Renisha McBride. Rebecca Cook/Reuters/Landov hide caption

itoggle caption Rebecca Cook/Reuters/Landov

Los Angeles police officers take a break during a basketball game with residents of the Nickerson Gardens housing project in July 2011. Violent crime at Nickerson Gardens and two nearby housing projects has fallen by almost half since 2010. Thomas Watkins/AP hide caption

itoggle caption Thomas Watkins/AP