In Colorado, it was the deadliest shooting since the Columbine High School massacre on April 20, 1999, when two students opened fire at the school in the Denver suburb of Littleton, killing 12 classmates and a teacher and wounding 26 others before killing themselves.
Below are some of the world's worst mass shootings:
- July 20, 2012: At least 12 people are killed and 58 are injured when a gunman enters an Aurora, Colo., movie theater, releases a canister of gas and then opens fire during opening night of the Batman movie "The Dark Knight Rises."
- July 22, 2011: Confessed mass killer Anders Behring Breivik kills 77 in Norway in twin attacks: a bombing in downtown Oslo and a shooting massacre at a youth camp outside the capital. The self-styled anti-Muslim militant admitted both attacks.
- Nov. 5, 2009: Thirteen soldiers and civilians were killed and more than two dozen wounded when a gunman walked into the Soldier Readiness Processing Center at Fort Hood, Texas, and opened fire. Army psychiatrist Maj. Nidal Hasan is charged with 13 counts of premeditated murder and 32 counts of attempted premeditated murder.
- April 30, 2009: Farda Gadyrov, 29, enters the prestigious Azerbaijan State Oil Academy in the capital, Baku, armed with an automatic pistol and clips. He kills 12 people before killing himself as police close in.
- March 10, 2009: Michael McLendon, 28, killed 10 people - including his mother, four other relatives, and the wife and child of a local sheriff's deputy - across two rural Alabama counties. He then killed himself.
- Sept. 23, 2008: Matti Saari, 22, walks into a vocational college in Kauhajoki, Finland, and opens fire, killing 10 people and burning their bodies with firebombs before shooting himself fatally in the head.
- Nov. 7, 2007: After revealing plans for his attack in YouTube postings, 18-year-old Pekka-Eric Auvinen fires kills eight people at his high school in Tuusula, Finland.
- April 16, 2007: Seung-Hui Cho, 23, kills 32 people and himself on Virginia Tech campus in Blacksburg, Va.
- April 26, 2002: Robert Steinhaeuser, 19, who had been expelled from school in Erfurt, Germany, kills 13 teachers, two former classmates and policeman, before committing suicide.
- April 20, 1999: Students Eric Harris, 18, and Dylan Klebold, 17, opened fire at Columbine High School in Littleton, Colo., killing 12 classmates and a teacher and wounding 26 others before killing themselves in the school's library.
- April 28, 1996: Martin Bryant, 29, bursts into cafeteria in seaside resort of Port Arthur in Tasmania, Australia, shooting 20 people to death. Driving away, he kills 15 others. He was captured and imprisoned.
- March 13, 1996: Thomas Hamilton, 43, kills 16 kindergarten children and their teacher in elementary school in Dunblane, Scotland, and then kills himself.
- Oct. 16, 1991: A deadly shooting rampage took place in Killeen, Texas, as George Hennard opened fire at a Luby's Cafeteria, killing 23 people before taking his own life. 20 others were wounded in the attack.
- June 18, 1990: James Edward Pough shoots people at random in a General Motors Acceptance Corp. office in Jacksonville, Fla., killing 10 and wounding four, before killing himself.
- Dec. 6, 1989: Marc Lepine, 25, bursts into Montreal's Ecole Polytechnique college, shooting at women he encounters, killing nine and then himself.
- Aug. 19, 1987: Michael Ryan, 27, kills 16 people in small market town of Hungerford, England, and then shoots himself dead after being cornered by police.
- July 12, 1976: Edward Charles Allaway, a custodian in the library of California State University, Fullerton, fatally shot seven fellow employees and wounded two others.
- Aug. 20, 1986: Pat Sherrill, 44, a postal worker who was about to be fired, shoots 14 people at a post office in Edmond, Okla. He then kills himself.
- July 18, 1984: James Oliver Huberty, an out-of-work security guard, kills 21 people in a McDonald's restaurant in San Ysidro, Calif. A police sharpshooter kills Huberty.
- Aug. 1, 1966: Charles Whitman opened fire from the clock tower at the University of Texas at Austin, killing 16 people and wounding 31.
The Associated Press contributed to this report.