People who carry out mass public shootings often display observable warning signs long before an attack, but those signals are frequently fragmented across friends, family members, coworkers and institutions, making them difficult to piece together, according to a new study from the Regional Gun Violence Research Consortium at the Rockefeller Institute of Government, a nonpartisan public policy think tank.

The report, which analyzed a sample of 171 mass public shootings in the United States between 1999 and 2024, such as those at workplaces, schools or shopping malls, found that these attacks are rarely sudden or unpredictable. Instead, researchers describe them as the result of cumulative stressors, concerning behaviors and communications of intent that, if connected, could offer opportunities for earlier intervention.

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An overwhelming majority of perpetrators, nearly 86%, communicated violent thoughts or intentions to at least one other person before carrying out an attack, a pattern researchers refer to as “leakage.” These disclosures most often occurred through in-person conversations or text messages and were typically made to people within the perpetrator’s immediate social circle, including friends, family members and coworkers.

On average, warning signs were spread across more than two different groups of observers, meaning no single person had a complete view of the escalating threat, according to the report.

The researchers also found that perpetrators tended to experience multiple overlapping stressors rather than a single triggering event. On average, people had five distinct stressors prior to an attack, including mental health challenges, job-related difficulties and family problems. Researchers also identified an average of 6.6 concerning behaviors per perpetrator, including suicidal ideation and other forms of emotional distress or aggression.

Planning often unfolded over an extended period. The report’s authors found that perpetrators spent an average of nearly 10 months preparing for attacks, including researching locations and studying prior mass shootings.

Firearms were most often obtained through legal channels, with nearly 60% purchased from federally licensed dealers. About one-third of perpetrators had at least one factor that would have legally prohibited them from possessing a firearm, according to the report.

Researchers also found that nearly two-thirds of perpetrators had prior contact with law enforcement, underscoring what they describe as missed opportunities for intervention when warning signs appeared across different systems but were not fully connected.

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“Warning signs are regularly present, observable, and known to people in the perpetrator’s social network long before the first shot is fired,” Jaclyn Schildkraut, the executive director of the consortium and lead author of the report, said in a news release. “By understanding how these indicators cluster and by building robust pathways for everyday bystanders to report what they see, we can connect the dots and intervene before a crisis turns into a tragedy.”

The report argues that improving communication between schools, law enforcement, mental health providers and community members could strengthen efforts to identify and respond to potential threats. It also highlights the need for clearer pathways for reporting concerning behavior and better systems for assessing risk when multiple warning signs emerge across different settings.

Alongside the findings, the consortium is developing an open-source database and training tools aimed at helping threat assessment professionals and community members recognize pre-attack behaviors and communication patterns.