Nearly three million Americans have served in Iraq and Afghanistan, and many of them came home angry.
A study published by Army psychiatrists in 2015 found "anger and aggression are among the most common issues reported by returning service members from combat deployments."
Shooters Micah Johnson and Gavin Long both served in the military and in war zones -- Johnson with an Army engineering unit in Afghanistan, and Long as a data specialist for the Marines in Iraq. There is no indication from their service records either saw front line combat, although in wars fought without front lines almost everybody could be exposed to some level of violence.
Johnson got out last year, a problem soldier accused of sexual harassment by a woman who specifically asked that he seek mental help. Long had been out of the Marines for six years, and had taken to expressing his anger in videos.
"You're in a world that's ran by devils," he said in one video. "Get this through your head right now. Devils run this."
The Marines say none of their studies dating back to the Vietnam war have found a conclusive link between combat deployment and anger.
But a 2013 Army study of 2,000 combat veterans found thirty-five percent getting angry enough to kick or smash something, twenty-two percent angry enough to threaten someone with physical violence and seven percent angry enough to actually hit someone.
A separate study pointed to something called "trait anger," defined as a propensity to become angry under stressful conditions. In other words, anger could have been a personality trait before they joined the military and the stress of service made it worse.
All soldiers returning from combat are screened for mental health problems, but according to the study those screenings do not include anger or aggression.