The terror of an armed gunman opening fire in public places has become an all-too-common occurrence, affecting American schools, churches, offices, holiday parties, government facilities and movie theatres.

While the attacks differ in location, level of carnage and the community impacted, an FBI study released Wednesday found that a common element of the attacks lies with the shooters, who are frequently motivated by grievances in their lives, wielding guns they obtained legally and targeting specific victims when they open fire.

The study, which examined dozens of active shooters between 2000 and 2013, found that contrary to the public perception of the episodes as being fuelled by mental health issues — an assertion frequently given voice by politicians, including U.S. President Donald Trump — law enforcement officials were only able to verify that about 25 per cent of the attackers had diagnosed mental health issues.

The attackers, who almost always were men or boys, typically attacked places that were familiar to them. They had acted in ways that concerned the people around them ahead of the attacks, with many expressing a desire to carry out violent acts. And most used guns they acquired legally, oftentimes buying the weapons specifically for their attacks, the study concluded.

“Offenders don’t snap,” Andre Simons, supervisory special agent of the FBI’s Behavioral Analysis Unit and a co-author of the study, said in a telephone interview. “They don’t wake up one morning and suddenly decide to attack.”

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Rather, the decision is part of “a long process,” Simons said. The study found that 77 per cent of attackers spent a week or longer planning their violence, which “does take some forethought and design on the part of the offender,” he said.

The new study, a copy of which was provided to The Washington Post before it was published, examined 63 cases, focusing on the shooters and the actions that led up to their attacks. It is a sequel of sorts to a 2014 report the FBI released examining active shootings during the same time frame. That report identified 160 incidents fitting the definition of “an individual actively engaged in killing or attempting to kill people in a confined and populated area” — and it found that such attacks had become more frequent during that 13-year period.

Since then, the bureau has continued studying shooting rampages, and its findings have been grim. In 2017, the massacres at a concert in Las Vegas and inside a church in Sutherland Springs, Tex., were among 30 active shootings the FBI found nationwide, the most it has tracked in a single year.

After attacks like those or the rampages at high schools this year in Parkland, Florida, and Santa Fe, Texas, attention often turns to what could have prevented the bloodshed and what red flags might have been missed. Other research has found that most attacks came after people close to the shooters noticed worrisome behaviour.

“Some of these concerning behaviours do presage violence,” said James Silver, a criminal justice professor at Worchester State University, one of the FBI study’s three authors.

The FBI study found that on average, each active shooter it examined “displayed four to five concerning behaviours over time.” About one in three shooters had made threats or confronted people they later targeted. More than half of them revealed their intentions to do something violent, a phenomenon called “leakage.”

The study’s authors write that there was “no single warning sign, checklist, or algorithm for assessing behaviours that” suggest a potential rampage shooter. But they say the study is meant in part to help members of the public keep an eye out for warning signs.

“One of the biggest findings for me is that there was one person in every active shooter’s life who noticed some sort of concerning behaviour,” said Sarah Craun of the FBI’s Behavioral Analysis Unit, the study’s third author. “Most of them had multiple concerning behaviours, between four and five. These were people who were already known to be struggling a little bit. There are actually some signs out there that people can see.”

The warning signs included threats and physical aggression, and the study found that most of the shooters “had a history of acting in an abusive, harassing, or oppressive way.” Others had abused their intimate partners or stalked people, highlighting again the recurrence of domestic violence in the lives of shooters before their violent attacks in public places.

The study’s authors say their work is unique among studies of rampage shooters because it relies “almost exclusively” on law enforcement files, rather than court records and media accounts. It examines a smaller pool of attacks than the 2014 study because it focused on the incidents where the FBI could obtain files with evidence including interviews with people who knew the shooters, personal writings or school and work records.

The files counter the claims that rampage shooters must be mentally ill. Trump has been among those giving voice to this belief after shootings, calling the alleged Parkland shooter “mentally disturbed” and the Sutherland Springs attacker a “mental health problem.” The study found confirmed diagnoses of mental illnesses in 25 per cent of the attackers, with some others exhibiting depression, anxiety and paranoia. But the authors noted that many Americans experience such symptoms, and say more thought should be given before concluding that a shooting has its roots in mental illness.

“Mental health does not automatically equate to violence, so we need to be very careful when we approach the motivations or the causes behind any active shootings,” Simons said. “Simply saying that all [shooters] must be mentally ill because it’s such a horrific and incomprehensible act is probably not accurate or helpful.”

Other research has similarly found tenuous links connecting shooting rampages and mental illness. In a 2015 study, Michael Stone, a clinical psychiatry professor at Columbia University, examined 235 people who carried out or attempted to carry out mass killings, and he concluded that about 22 per cent could be considered mentally ill. An article published the same year in the academic journal Annals of Epidemiology found that “the large majority of people with mental disorders do not engage in violence against others, and that most violent behaviour is due to factors other than mental illness.”

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Experts have said that what mass killers often share is a sense of victimization and believe they are being persecuted or treated unfairly. The new FBI study reported similar findings. Most of the shooters examined were fuelled by a grievance that “may not have been reasonable or even grounded in reality, but it appeared to serve as the rationale for the eventual attack, giving a sense of purpose to the shooter.”

In some cases, this grievance could stem from something like losing a job or being romantically rejected. Silver, the criminal justice professor, said most people would be upset in those situations, but their strongest feelings eventually dissipate.

“It’s the people for whom it doesn’t dissipate, and the people for whom that action, which may seem reasonable to everyone else in the world, becomes for the active shooter a cause, in that they have been unfairly, unjustly wronged,” Silver said. “It’s not the action itself, it’s their reaction to it that turns it into a grievance.”