Mourners gather in Littleton, Colo., on April 20, 2021, to remember the victims of the 1999 Columbine High School shooting.
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Journalist Mark Follman says that understanding the psychology of shooters and intervening where appropriate can help prevent massacres from happening. His new book is Trigger Points.
Mourners gather in Littleton, Colo., on April 20, 2021, to remember the victims of the 1999 Columbine High School shooting.
Michael Ciaglo/Getty Images
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