(CNN) - Each attack is as unique as it is horrific. But among the country's deadliest acts of violence last year, investigators found similarities between many of the alleged perpetrators.
The Secret Service's annual Mass Attacks in Public Spaces report, released Tuesday, says almost all of the alleged criminals in 27 mass attacks last year had experienced a major stressor in their lives.
And almost all had made alarming or threatening communications directed toward or in the presence of others. In more than 75% of the cases, someone else had noticed a sign of concern.
The findings emphasize the need for anyone who hears a threat to speak up, said acting Homeland Security Secretary Kevin McAleenan.
He said prevention is a "community effort."
"While not every act of violence can be prevented, the MAPS report indicates we can do much more to prevent targeted violence together when appropriate systems are in place," he said.
The report was published by the Secret Service's National Threat Assessment Center. The 27 attacks studied involved those in which three or more people were killed or injured last year. They include attacks in workplaces, schools and places of worship.
Half of the attackers were motivated by a grievance, the report found.
"In this report, two-thirds had mental health issues that deeply impacted their relationships," said Dr. Lina Alathari, chief of the National Threat Assessment Center.
"For some of them, it contributed to the motive of why they carried out the attack."
But the motives varied significantly. Some alleged attackers were motivated by a "perceived wrong they felt was done to them with domestic issues; grievance with a spouse or intimate partner; workplace issues or other disgruntled issues; or personal issues, such as losing a video game competition or getting into an argument with a manager of a retail establishment," Alathari said.
Only two of the attacks had an ideological component: one in which an anti-abortion perpetrator who attacked a Planned Parenthood clinic, and the attack at a synagogue stemming from white supremacy beliefs.
"While there is no single profile of a person who commits targeted violence, this report and so many others ... aims to assist our partners in law enforcement, in the education area, and so many other stakeholders in understanding some of the motivations behind mass attacks and their causative factors," Secret Service Director James Murray said.
McAleenan said the Secret Service has a "unique capability" in helping prevent mass attacks because the agency has the most experience in protecting public spaces.
Alathari said the study's findings can help encourage the public to report concerning behavior or threats.
"We want to be able to identify these individuals before they embark on that path that makes them think violence is an option," she said.
"We want the community to know that prevention is everyone's responsibility. We all have a role in that, it's not just law enforcement alone. So they need to trust their gut instinct."
CNN's Geneva Sands and Christian Sierra contributed to this report.
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