The FBI defines mass murder as a massacre in which four or more people are killed in a single event. Incidences have tripled over the past decade.

There have been 16 mass killings with double-digit victim counts since 1966 -- the year that Charles Whitman murdered 12 people at the University of Texas, bringing massacres to the forefront of public consciousness. 

Sources: James Alan Fox and Jack Levin, Extreme Killing: Understanding Serial and Mass Murder, 2nd Ed. Thousand Oaks, CA: Sage, 2012; Citizens Crime Commission of New York City, The New York Times and Mother Jones.