With today's deadly school shooting in Nevada, look back at other major school shootings in U.S. history.
Dec. 14, 2012: Gunman Adam Lanza massacred 20 first-graders and six staff members at Sandy Hook Elementary School in Newtown, Conn. He shot his mother to death before going to the school, and killed himself as authorities arrived.
iStock / gbh007
National Portrait Gallery, London
iStock / mashabuba
iStock / szefei
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