This article talks about mass shooters. They never just snap. They plan for a long time.
http://www.motherjones.com/politics/2015/09/mass-shootings-threat-assessment-shooter-fbi-columbine
happens at a school, an office building, or a movie theater, the question will again be asked: "What made him snap?" But mass murder is not an impulsive crime. Virtually every one of these attacks, forensic investigations show, is a predatory crime, methodically planned and executed. Therein lies the promise of threat assessment: The weeks, months, or even years when a would-be killer is escalating toward violence are a window of opportunity in which he can be detected and thwarted.
A growing body of research has shed light on this "pathway to violence." It often begins with an unshakable sense of grievance, which stirs thoughts about harming people and leads to the planning and preparation for an attack. Elliot Rodger, convinced that women were unfairly denying him sex, seethed for months and fantasized about a "day of retribution" before he bought firearms, scouted sorority houses, and went on to kill 6 people and injure 14 others near Santa Barbara, California, in May 2014.
A confluence of behaviors can indicate that someone is poised to walk into a school or a shopping mall and open fire. These include an obsession with weapons, a fixation on images of violence, and a history of aggressive acts that aren't directly related to the planned attack—possibly a way for the perpetrator to test his resolve. Almost a year before Rodger struck, he attempted to push some women off a 10-foot ledge at a house party. Some killers have mutilated pets before going on rampages.
In fact, the vast majority of mass shooters signal their intentions in advance, though usually not directly to their intended targets. This "leakage," as threat assessment teams call it, can be difficult to recognize. Before Dylann Roof murdered nine black churchgoers in Charleston, South Carolina, he told a friend about his desire to kill people and start a race war. (The friend claimed he didn't think Roof was serious.) Weeks before Rodger attacked, he posted disturbing videos that prompted his mother to alert a county mental health agency that he was suicidal. In response, sheriff's deputies went to Rodger's apartment to do a welfare check, interviewing him just a few strides from where he'd stashed three handguns and hundreds of rounds of ammunition. He persuaded them he was fine. "Thankfully, all suspicion of me was dropped," he later wrote, "and the police never came back."
