However, when people with military backgrounds “radicalise, they tend to radicalise to the point of mass violence,” said START’s Michael Jensen, who leads the team that has spent years compiling and vetting the dataset.

His group found that among extremists “the No. 1 predictor of being classified as a mass casualty offender was having a U.S. military background – that outranked mental health problems, that outranked being a loner, that outranked having a previous criminal history or substance abuse issues.”