What separates the older mass shooters from their younger counterparts is that mass shooters in their 20s and 30s typically study previous mass shooters for inspiration and validation. Younger shooters also tend to communicate intent to do harm in advance. This practice, known as leakage, is often seen as a final cry for help. Younger shooters also tend to leave behind manifestos to communicate their anger and grievances to the world, the data shows. Analysis of their expressed motives suggest they are seeking fame and notoriety for their actions.

None of the shooters aged 60 and above in our database did that – although investigations are ongoing in the back-to-back California cases. Instead, they tend to have experienced a recent stressor, such as a family conflict or debt. They are more likely to be motivated by legal, financial and interpersonal conflicts, not hate or fame-seeking like many of their younger counterparts.

But all perpetrators of mass shootings, young and old, have some things in common. Their mass shooting is intended to be their final act. Whether they die by suicide – as is seemingly the case with the alleged Monterey Park shooter, are killed on scene, or sit and wait to be arrested like the Half Moon Bay suspect did, mass shootings are a final act of hopelessness and anger.

They also have access to the firearms they need to commit these devastating crimes.

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Monterey Park: A pioneering Asian American suburb shaken by the tragedy of a mass shooting

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Jillian Peterson receives funding from the National Institute of Justice and the Joyce Foundation.

James Densley has received funding from the National Institute of Justice and the Joyce Foundation.