Mourners gather at a makeshift memorial left in memory of the victims killed in a shooting at Santa Fe High School in Santa Fe, Texas, U.S., May 21, 2018.
Mourners gather at a makeshift memorial left in memory of the victims killed in a shooting at Santa Fe High School in Santa Fe, Texas, U.S., May 21, 2018.

There are many theories about what motivates mass shooters.One common one holds that every active shooter is mentally ill.

But a new study by the FBI calls that notion into question, showing that the vast majority of mass shooters do not have a diagnosed mental illness.

The study, released Wednesday, looked at 63 active shooters between 2000 and 2013, focusing on the shooters' behaviors prior to their attacks.

While most shooters experienced mental health "stressors" such as anxiety and depression, the researchers found that only 25 percent had been diagnosed by a health professional with a mental illness "of any kind" prior to an attack. Of those diagnosed with a mental illness, three had a psychotic disorder, a condition that is sometimes associated with violence.

This is not the first study to de-link mass shootings from mental illness, but the notion that all mass shooters are mentally ill persists, and the FBI says it is "misleading and unhelpful."

Social, contextual factors

"In light of the very high lifetime prevalence of the symptoms of mental illness among the U.S. population, formally diagnosed mental illness is not a very specific predictor of violence of any type, let alone targeted violence," the researchers wrote. "Therefore, absent specific evidence, careful consideration should be given to social and contextual factors that might interact with any mental health issue before concluding that an active shooting was 'caused' by mental illness."

A hearse carrying the casket of Aaron Feis, one of
FILE - A hearse carrying the casket of Aaron Feis, one of the victims of the mass shooting at Marjory Stoneman Douglas High School, drives past a U.S. flag, placed on a firetruck, during his funeral in Coral Springs, Fla., Feb. 22, 2018.

The findings come amid an epidemic of mass shootings in the United States. Last year saw a record 30 mass shootings in the United States, according to the FBI. (The FBI defines a mass killing as the killing of three of more people in a public place.) This year, there have been several high-profile mass killings, including one that left 17 people dead at a Florida high school in February and another last month that killed 10 people at a high school in Texas.

The mass killings have led to calls for not just stricter gun laws but also for improved mental health care for people who may be contemplating acts of violence. Others have shied away from tying mass shootings to mental illness, partly out of fear of stigmatizing people who are mentally ill.

But if mental illness is not a reliable predictor of an active shooter in the making, there are other "concerning behaviors" that would-be shooters display in the lead-up to an attack, the FBI researchers say.

Signals of 'impending violence'

These range from potential symptoms of a mental health disorder, interpersonal interactions, recklessness, consumption of violent media and physical aggression. The study found that all but three of the shooters exhibited at least two such behaviors before an attack.

"In the weeks and months before an attack, many active shooters engage in behaviors that may signal impending violence," the researchers wrote. "While some of these behaviors are intentionally concealed, others are observable and — if recognized and reported — may lead to a disruption prior to an attack."

Contrary to a common perception, active shooters are not "completely isolated" from society and "had at least some social connection to another person," the researchers wrote.

The study also showed that a majority of active shooters obtain their firearms legally. In 40 percent of the cases, the shooter purchased a firearm legally with the specific purpose of carrying out an attack. Thirty percent already possessed firearms. Just 2 percent purchased firearms illegally, while 6 percent stole them, the study found.