(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."

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