April 20 will be the 20th anniversary of the massacre at Columbine High School. Mother Jones yesterday published a report on mass shooters (or would-be shooters) who in one way or another mentioned the Columbine murders as something they wanted to emulate, or as a benchmark they hoped to outdo. Since 1999, there have been more than 100 plots or actual attacks influenced by Columbine. MoJo national affairs editor Mark Follman notes,

And those are just the cases for which there is some kind of public record: In talking with law enforcement and mental health sources who work to prevent such attacks, I've learned of dozens more Columbine-influenced threats that never made the news.