Half Moon Bay resident Susana Gutierrez visits a memorial for shooting victims at Mac Dutra Park in Half Moon Bay, California. Image Credit: Reuters

Half of mass attackers in the United States were retaliating for personal, domestic or workplace-related grievances, and many used firearms that were obtained legally, according to a Secret Service report published days after a man in California shot and killed at least seven people in what authorities believe could have been an act of "workplace violence."

The report, released by the Secret Service National Threat Assessment Center on Wednesday, examined 173 targeted attacks carried out by 180 perpetrators between 2016 and 2020 in public or semipublic locations. It defines a mass attack as an act of violence in which three or more people, excluding the perpetrator, were killed or injured.

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The researchers found that three-quarters of the perpetrators had displayed behaviors or communicated in a way that concerned others before the attack. About 29 percent of attackers were described either by themselves or others as "withdrawn, loners or anti-social," the report said, and more than half had experienced mental health symptoms before the attack.

Ron Avi Astor, a professor at the University of California at Los Angeles, who studies school violence, said suicidal ideation is among the most important mental health-related factors he considers when examining mass attackers in general, and mass shooters in particular.

"We often frame it as a depression and other kinds of mental health issues when it should really be about suicidal ideation," said Astor, who was not involved with the report. "A good number of them are suicidal, a good number of them are trying to create terror, and ... some of them might want to be remembered when they're gone."

About 93 percent of the attackers studied experienced at least one significant life stressor within five years of the incident, according to the findings. Most of them - 77 percent - experienced the stressors, which ranged from demotion at work to homelessness and divorce, within a year.

96 percent of the perpetrators studied in the report were assigned male at birth, and 47 percent were White, non-Hispanic individuals. One-third of the perpetrators were Black, and the average age for the attackers was 34.

The United States has an epidemic of gun violence, and the report found that 73 percent of attacks involved the use of one or more firearms. Illegally obtained firearms were present in nearly one-quarter of attacks involving firearms.

Lori Ann Post, who studies mass killings as a public health issue at Northwestern University, said mass shooters ages 26 and older tend to be acting in retaliation for a perceived loss or failure. "The older ones seem to have left home, they've separated from their parents, but then they self-implode and destroy their life," she said. "They want revenge, they're disgruntled, they want payback."

She said the report should have more narrowly defined mass attacks as incidents in which a perpetrator killed four or more people, because such attackers tend to be more organized and thoughtful in their planning. (The Gun Violence Archive defines mass shootings as incidents in which four or more people, excluding the shooter, are killed or injured. Washington Post reporters are usually dispatched to cover an incident when there are at least four deaths.)

"Using the threshold of 3 or more injured is going to generate so many more cases, and it doesn't tell us what the problem is," said Post, who was not affiliated with the report.

Meanwhile, about 26 percent of the attackers expressed beliefs that involved conspiracy theories or hateful ideologies.

"Bias-based beliefs included antisemitism, anti-immigrant, anti-LGBTQ, anti-Asian, misogyny, and race-based supremacy," the authors wrote, adding that some perpetrators also subscribed to anti-government and extreme Islamist ideologies.

The database that Post uses for her research has recorded at least six mass shootings in the United States, with 39 people killed and 11 injured so far this year. The Gun Violence Archive puts the number of mass shootings at 39 or more.

"No matter who you are, what data you use, there is an escalation going on," she said.