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.
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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.
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.