On April 20, 1999, two young men fatally shot 13 people and injured 24 others at Columbine High School in Littleton, Colorado. A Mother Jones investigation shows that the nation’s deadliest high school shooting has since inspired at least 74 plots or attacks across 30 states. To gauge just how deep the problem goes, we examined scores of news reports and public documents and interviewed multiple law enforcement officials.

The data we have compiled reveals some disturbing patterns. In at least 14 cases, the Columbine copycats aimed to attack on the anniversary of the massacre. Individuals in 13 cases indicated that their goal was to outdo the Columbine body count. In at least 10 cases, the suspects and attackers referred to the pair who struck in 1999, Eric Harris and Dylan Klebold, as heroes, idols, martyrs, or God. And at least three perpetrators made pilgrimages to Columbine High School from other states.

As one longtime security specialist explains in our investigation into a growing national effort to stop mass shooters before they strike, “It’s a cult following unlike anything I’ve ever seen before.”

Mother Jones is not publishing our research on Columbine copycats beyond the numbers and analysis below. Though much of the case-level details we’ve collected are publicly available, we have chosen not to make them easily accessible in one place, where they might potentially be used by would-be copycats searching for inspiration or information. For more of our reporting on the copycat problem stemming from Columbine and other high-profile attacks, read the main investigation here.