Some of world's worst mass shootings:
__ July 20, 2012: At least 12 people are killed 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: At least 80 people are killed at a summer camp on the Norwegian island of Utoya. A man arrested also is suspected in a blast earlier the same day in downtown Oslo that killed seven. Anders Behring Breivik, 33, has admitted to carrying out both attacks. He awaits sentencing on Aug. 24.
— 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.