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

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This image provided by the Durham County Sheriff's Office shows a booking photo of Craig Stephen Hicks, 46, who was arrested on three counts of murder early Wednesday. On his Facebook page, Hicks described himself as a gun-toting atheist. Durham County Sheriff's Office/AP hide caption

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Mindy Corporon speaks during a news conference, flanked by Will Corporon (left) and Tony Corporon, at their church in Leawood, Kan., on Monday. Their father, Dr. William Corporon, and Mindy Corporon's 14-year-old son were killed during Sunday's shooting at the Jewish Community Center in Overland Park, Kan. Orlin Wagner/AP hide caption

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Frazier Glenn Cross, also known as Glenn Miller, is accused of killing three people Sunday in Kansas City. He allegedly attacked them at a Jewish community center and a Jewish retirement facility. Johnson County, Kan., Sheriff's Office hide caption

itoggle caption Johnson County, Kan., Sheriff's Office