"The shooting in San Bernardino was unlike nearly every other shooting of its type in the United States in the past decade and a half because it involved more than one gunman and the suspects managed to flee the scene.
Just two of 160 active shooter episodes from 2000 to 2013 had more than one gunman, according to a 2014 report released by the F.B.I. Only 25 of the gunmen got away without being arrested or killed, or committing suicide.
The study analyzed episodes involving active gunmen, where a shooting was in progress when law enforcement officers responded.
“Humans in general are adverse to killing other individuals, so it’s hard to sell someone else on helping you with that,” said J. Pete Blair, a criminal justice professor at Texas State University who worked on the F.B.I. study. “Oftentimes the person is socially isolated and not socially successful, and that creates a situation in which you are on your own and it’s hard to get others to buy into your vision or anger."
NEW YORK TIMES