SANDY HOOK, Conn.—A man armed with two semiautomatic pistols killed 20 children at an elementary school Friday in a rampage that left 27 people dead and one injured in this suburban town before he took his own life, authorities said.

Police said the suspect, 20-year-old Adam Lanza, entered Sandy Hook Elementary School and began firing shortly after 9:30 a.m., concentrating on two classrooms. It was the second-worst campus shooting in U.S. history.

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