Anders Behring Breivik is the admitted perpetrator of the July 2011 attacks in Norway that killed 77 people.
Student Seung-Hui Cho shot and killed 30 people on Virginia Tech's campus in 2007. The mass murder ended when he turned the gun and shot himself in the head.
In 1966, Thomas Hamilton attacked a classroom at Dunblane Primary School in the U.K.
Mass murderer Eric Harris and his friend Dylan Klebold killed themselves, 13 people and wounded more than 20 others at Columbine High School on April 20, 1999.
Andrew Kehoe was a mass murderer who went on a 1927 killing spree that included dynamiting the Bath, Michigan Consolidated School, killing 37 children.
Along with Eric Harris, Dylan Klebold shot and killed 13 people and injured 20 at Columbine High School on April 20, 1999.
Adam Lanza shot and killed 20 first-graders and six adults at Sandy Elementary School in Newtown, Connecticut, on December 14, 2012, before shooting himself.
Marc Lepine went on a shooting spree in 1989 that became known as the Montreal Massacre, which killed 14 people.
Lee Boyd Malvo was one of the vicious snipers who bred fear in the Washington, D.C. area for more than 20 days in 2002.
John Allen Muhammad became an infamous figure as part of a sniper team that terrorized the Washington, DC, area for several weeks in October 2002
In 1966, Richard Speck committed one of the most horrifying mass murders in American history when he brutalized and killed eight student nurses living on Chicago's South Side.
After murdering a gas station attendant in 1957, Nebraska native Charles Starkweather embarked on a murderous rampage with girlfriend Caril Ann Fugate in 1958. Together, they killed 10 people.
Charles Whitman was a former Marine and sniper who killed 16 people at the University of Texas in 1966.