In the weeks after a pair of massacres in El Paso, Texas, and Dayton, Ohio, left 31 dead, local police and federal authorities scrambled to contain a succession of chilling new threats.

A Florida man allegedly vows to "break a world record" for mass shooting casualties; a disgruntled hotel cook in California threatens to transform a Marriott lobby into a killing field; a Jewish community center in Ohio is the target in a suspected shooting plot.

Police stopped each one before anyone was harmed. But the arrests, spanning just over a week, highlighted a frequent theme in the government's efforts to prevent domestic terrorism and other forms of mass violence: law enforcement didn't see the potentially deadly storms approaching until members of the public stepped forward with crucial information, and authorities had little power to intercede until an attack appeared imminent.

Read more: USA Today