Studies have shown, for example, that in school shootings, the killers virtually always “leak” their intentions, leaving a trail of clues behind them. Nikolas Cruz, the 19-year-old who the police said has confessed in the Parkland shooting, apparently was no exception: Students reportedly avoided him and joked that if anyone were going to shoot up the school, it would be him.

Researchers have also found that in many, if not most, cases of school violence, the perpetrator has done extensive research on previous school shootings, studying them in detail, often with special attention to the killings at Columbine High School in 1999. A study of nine school shootings in Europe conducted by J. Reid Meloy, a forensic psychologist in San Diego who consults on threat assessment for schools and corporations, found that a third of the killers had “consciously imitated and emulated what had happened in Columbine.”

Finally, there is nascent, but increasing, evidence that violence begets violence, with one school shooting — especially if it receives a lot of publicity — leading to others, a phenomenon that researchers refer to as “contagion.”