The FBI says it has identified a series of behavioural signs that may flag a person as a potential mass shooter, and allow police to ward off killing sprees.
Mass shooters are mostly white men suffering from various stress factors, a belief that they are victims of some injustice and who pick targets that they are familiar with, the FBI report said.
Officers studied 63 shootings that resulted in at least one death that were committed between 2000 and 2013, and which allowed FBI behavioural experts to determine how the shooter acted in the run-up to the killing.
The report challenges a widely held belief that shooters are suffering from a mental illness or extreme social isolation, two reasons frequently cited as triggers for an attack.
According to the FBI, three-quarters of the killers they studied were not suffering from any diagnosable mental illness.
"Declarations that all active shooters must simply be mentally ill are misleading and unhelpful," the report said.
After the shooting at a school in Parkland, Florida, in February that left 17 people dead, numerous politicians, including President Donald Trump and the gun lobby, said that the gunman -- a former student -- had suffered psychological problems and that his behaviour been reported to the authorities.
The FBI said that most of the shooters "experienced multiple stressors" such as financial or marital problems, or professional and personal disputes before launching their attacks.
That often results in "concerning behaviours" such as depression, paranoia, violent acts and direct or indirect threats, as well as skipping work or difficulty in communicating with others.
In the 63 cases studied, this behaviour was detected by at least one person close to the shooter, but fewer than half of them -- just 41 percent -- reported such behaviour to the authorities.
"Early recognition and reporting of concerning behaviours to law enforcement or threat assessment professionals may initiate important opportunities for mitigation," the FBI said.
The report admitted however that "in retrospect certain facts may take on a heightened degree of significance that may not have been clear at the time," and said the checklist of signs was more of a guideline than a predictor of future violence.
"What emerges is a complex and troubling picture of individuals who fail to successfully navigate multiple stressors in their lives while concurrently displaying four to five observable, concerning behaviours, engaging in planning and preparation, and frequently communicating threats or leaking indications of an intent to attack," the report said.
"As an active shooter progresses on a trajectory towards violence, these observable behaviours may represent critical opportunities for detection and disruption.
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