-- 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: 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.

Related on HuffPost:

Loading Slideshow...
  • Tom Sullivan , holds a photograph of his son, Alex Sullivan, as he pleads with the media to help him find his son, outside Gateway High School on Friday, July 20, 2012 in Aurora, Colo. Alex Sullivan, was celebrating his 27th birthday by attending midnight premiere of the Batman movie Friday night. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • A woman cries outside Gateway High School where witness were brought for questioning after a gunman opened fire at a midnight premiere of The Dark Knight Rises Batman movie Friday, July 20, 2012 in Aurora, Colo. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Tom Sullivan, center, embraces family members outside Gateway High School where he has been searching frantically for his son Alex Sullivan who celebrated his 27th birthday by going to see "The Dark Knight Rises," movie where a gunman opened fire Friday, July 20, 2012, in Aurora, Colo. (Photo credit: AP Photo/Barry Gutierrez)

  • Eyewitness Chandler Brannon, 25, sits outside Gateway High School where witnesses were brought for questioning after a shooting at a movie theater showing the Batman movie "The Dark Knight Rises," Friday, July 20, 2012 in Aurora. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • A small group prays outside Gateway High School where witness were brought for questioning Friday, July 20, 2012 in Aurora, Colo. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Family and friends wait outside Gateway High School where witnesses were brought for questioning after a shooting at a movie theater showing the Batman movie "The Dark Knight Rises," Friday, July 20, 2012 in Aurora, Colo. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Eyewitness Jacob Stevens, 18, hugs his mother Tammi Stevens after being interview by police outside Gateway High School where witnesses were brought for questioning Friday, July 20, 2012 in Aurora, Colo. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Eyewitness Isaiah Bow hugs his mother Shamecca Davis after being questioned by police outside Gateway High School where witnesses were brought in, Friday, July 20, 2012 in Aurora, Colo. After leaving the theater Bow went back in to find his girlfriend. "I didn't want to leave her in there. But she's ok now," Bow said. A gunman wearing a gas mask set off an unknown gas and fired into the crowded movie theater killing 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Emma Goos, 19, hugs her mother, Judy Goos, outside Gateway High School where witnesses were brought for questioning Friday, July 20, 2012, in Aurora, Colo. Emma was in the third row of the theater of the new Batman movie when the shooter entered. She helped apply pressure to a man's head who was injured. (Photo credit: AP Photo/Barry Gutierrez)

  • Police cars in front of the Century 16 theater in Aurora, Colorado where a gunman opened fire during the opening of the new Batman movie "The Dark Knight Rises" killing at least 15 people and wounding 50 others on the morning of July 20, 2012. The shooting suspect in custody after at least 12 people were shot dead and around 50 were wounded at a movie screening in the United States has been identified as 24-year-old James Holmes, US media said Friday. Television networks cited the FBI as saying that Holmes, from the town of Aurora, Colorado, scene of the midnight shooting at a screening of the Batman movie "The Dark Knight Rises," did not appear to have any known terrorism connections. (Photo credit: AFP PHOTO/JONATHAN CASTNER JONATHAN CASTNER/AFP/GettyImages)

  • Police tape cordons off the parking area around the Century 16 movie theater in Aurora, Colorado, July 20, 2012 where a gunman opened fire during the showing of the new Batman movie. At least 12 people were killed and around 50 wounded in the cinema shooting. (Photo credit: AFP PHOTO JONATHAN CASTNERJONATHAN CASTNER/AFP/GettyImages)

  • Police cars in front of the Century 16 theater in Aurora, Colorado where a gunman opened fire during the opening of the new Batman movie "The Dark Knight Rises" killing at least 15 people and wounding 50 others on the morning of July 20, 2012. The shooting suspect in custody after at least 12 people were shot dead and around 40 were wounded at a movie screening in the United States has been identified as 24-year-old James Holmes, US media said Friday. Television networks cited the FBI as saying that Holmes, from the town of Aurora, Colorado, scene of the midnight shooting at a screening of the Batman movie "The Dark Knight Rises," did not appear to have any known terrorism connections. (Photo credit: AFP PHOTO/JONATHAN CASTNERJONATHAN CASTNER/AFP/GettyImages)

  • Police cars in front of the Century 16 theater in Aurora, Colorado where a gunman opened fire during the opening of the new Batman movie "The Dark Knight Rises" killing at least 15 people and wounding 50 others on the morning of July 20, 2012. The shooting suspect in custody after at least 12 people were shot dead and around 50 were wounded at a movie screening in the United States has been identified as 24-year-old James Holmes, US media said Friday. Television networks cited the FBI as saying that Holmes, from the town of Aurora, Colorado, scene of the midnight shooting at a screening of the Batman movie "The Dark Knight Rises," did not appear to have any known terrorism connections. (Photo credit: AFP PHOTO/JONATHAN CASTNERJONATHAN CASTNER/AFP/GettyImages)

  • Police cars are seen in the parking area around the Century 16 movie theater in Aurora, Colorado, July 20, 2012 where a gunman opened fire during the showing of the new Batman movie. At least 12 people were killed and around 50 wounded in the cinema shooting. (Photo credit: AFP PHOTO JONATHAN CASTNERJONATHAN CASTNER/AFP/GettyImages)

  • Police use a video camera to look inside an apartment where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 12 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (Photo credit: AP Photo/Ed Andrieski)

  • Police break out a window of an apartment where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 12 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (Photo credit: AP Photo/Ed Andrieski)

  • Shamecca Davis hugs her son Isaiah Bow, who was an eye witness to the shooting, outside Gateway High School where witness were brought for questioning Friday, July 20, 2012 in Denver. After leaving the theater Bow went back in to find his girlfriend. " I didn't want to leave her in there. But she's ok now," Bow said. A gunman wearing a gas mask set off an unknown gas and fired into a crowded movie theater at a midnight opening of the Batman movie "The Dark Knight Rises," killing at least 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • Judy Goos, second from left, hugs her daughters friend, Isaiah Bow, 20, while eye witnesses Emma Goos, 19, left, and Terrell Wallin, 20, right, gather outside Gateway High School where witness were brought for questioning Friday, July 20, 2012 in Denver. A gunman wearing a gas mask set off an unknown gas and fired into a crowded movie theater at a midnight opening of the Batman movie "The Dark Knight Rises," killing at least 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • A SWAT team officer stands watch near an apartment house where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 14 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (Photo credit: AP Photo/Ed Andrieski)

  • Police gather near an apartment house where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 12 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (Photo credit: AP Photo/Ed Andrieski)

  • A SWAT team officer stands watch near an apartment house where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 12 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (Photo credit: AP Photo/Ed Andrieski)

  • Jacob Stevens, 18, hugs his mother Tammi Stevens after being interview by police outside Gateway High School where witness were brought for questioning after a shooting at a movie theater, Friday, July 20, 2012 in Denver. A gunman wearing a gas mask set off an unknown gas and fired into a crowded movie theater at a midnight opening of the Batman movie "The Dark Knight Rises," killing at least 12 people and injuring at least 50 others, authorities said. (Photo credit: AP Photo/Barry Gutierrez)

  • (Credit: Sky News)

  • (Credit: Sky News)

  • (Credit: Sky News)

  • (Credit: Sky News)

  • Police are pictured outside of a Century 16 movie theatre where as many as 12 people were killed and many injured at a shooting during the showing of a movie at the in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • Police are pictured outside of a Century 16 movie theatre where as many as 12 people were killed and many injured at a shooting during the showing of a movie at the in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • Aurora Police Chief Daniel Oates talks to media at Aurora Mall where as many as 12 people were killed and many injured at a shooting at the Century 16 movie theatre in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • Aurora Police Chief Daniel Oates talks to media at the Aurora Mall where as many as 14 people were killed and many injured at a shooting at the Century 16 movie theatre in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • Police are pictured outside of a Century 16 movie theatre where as many as 12 people were killed and many injured at a shooting during the showing of a movie at the in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • Police are pictured outside of a Century 16 movie theatre where as many as 12 people were killed and many injured at a shooting during the showing of a movie at the in Aurora, Colo., Friday, July 20, 2012. (Photo credit: AP Photo/Ed Andrieski)

  • People gather outside the Century 16 movie theatre in Aurora, Colo., at the scene of a mass shooting early Friday morning, July 20, 2012. Police Chief Dan Oates says 12 people are dead following the shooting at the suburban Denver movie theater. He says 50 others were injured when gunfire erupted early Friday at the Aurora theater. Oates says a gunman appeared at the front of one of the Century 16 theaters. <em>Photo Credit: Karl Gehring, <a href="http://www.huffingtonpost.com/news/exposure/" target="_hplink">Denver Post</a>. </em> / AP

  • (Photo credit: 7NewsMorning/Twitter)

  • (Photo credit: 7NewsMorning/Twitter)

  • (Photo credit: Faunte Thompson/Twitter)

  • (Photo credit: 7NewsMorning/Twitter)

  • (Photo credit: Brandon Rittiman/Twitter)

  • (Photo credit: 7NewsMorning/Twitter)

  • (Photo credit: Jakeob Bame/Twitter)