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Eleven schools since Columbine High School in 1999 have had mass shootings. Accounts by witnesses and survivors are eerily similar. (Video: Monica Akhtar/The Washington Post, Photo: Matt McClain/The Washington Post)

The stunning number swept across the Internet within minutes of the news Wednesday that, yet again, another young man with another semiautomatic rifle had rampaged through a school, this time at Marjory Stoneman Douglas High in South Florida.

The figure originated with Everytown for Gun Safety, a nonprofit group, co-founded by Michael Bloomberg, that works to prevent gun violence and is most famous for its running tally of school shootings.