Here is a timeline of some of the worst shooting incidents carried out by one or two gunmen around the world in the last 20 years:
March 13, 1996: Britain - Gunman Thomas Hamilton bursts into a primary school in the Scottish town of Dunblane and shoots dead 16 children and their teacher before killing himself.
April 28, 1996: Australia - Martin Bryant unleashes modern Australia's worst mass murder when he shoots dead 35 people at the Port Arthur tourist site in the southern state of Tasmania.
April 1999: United States - Two heavily armed teenagers go on a rampage at Columbine High School in Littleton, Denver, shooting 13 students and staff before taking their own lives.
July 1999: United States - A gunman kills nine people at two brokerages in Atlanta, after apparently killing his wife and two children. He commits suicide five hours later.
June 2001: Nepal - Eight members of the Nepalese Royal family are killed in a palace massacre by Crown Prince Dipendra who later turns a gun on himself and dies few days later. His youngest brother also died later, raising the death toll to 10.
April 26, 2002: Germany - In Erfurt, eastern Germany, 19-year-old Robert Steinhauser opens fire after saying he was not going to take a math test. He kills 12 teachers, a secretary, two pupils and a policeman at the Gutenberg Gymnasium, before killing himself.
October 2002: United States - John Muhammad and Lee Malvo kill 10 people in sniper-style shooting deaths that terrorize the Washington DC area.
Nov. 21, 2004: United States - Six people are killed when Chai Soua Vang, a 35-year-old Hmong immigrant and naturalized U.S. citizen, shoots eight people while deer hunting east of Birchwood in northern Wisconsin. A truck driver from St Paul, Minn, Vang is sentenced to six consecutive life terms in prison.
March 12, 2005: United States - Seven people are killed and four wounded when Terry Michael Ratzmann opens fire at a Living Church of God service at the Sheraton Hotel in Brookfield, Wisconsin. Ratzmann, a 44-year-old computer technician, the commits suicide.
April 16, 2007: United States - Virginia Tech, a university in Blacksburg, Virginia, becomes site of the deadliest rampage in US history when a gunman kills 32 people and himself.
Oct 7, 2007: United States - Six people are killed and one wounded when Sheriff's department deputy Tyler James Peterson goes on a shooting rampage at his ex-girlfriend's apartment, in Crandon, Wisconsin. Peterson, 20, then shot and killed himself.
Nov 7, 2007: Finland - Pekka-Eric Auvinen kills six fellow students, the school nurse, the principal and himself with a handgun at the Jokela High School near Helsinki.
Sept 23, 2008: Finland - Student Matti Saari opens fire in a vocational school in Kauhajoki in northwest Finland, killing nine other students and one male staff member before killing himself.
March 11, 2009: Germany - A 17-year-old gunman dressed in combat gear kills nine students and three teachers at a school near Stuttgart. He also kills one other person at a nearby clinic. He was later killed in a shoot-out with police. Two additional passers-by were killed and two policemen seriously injured, bringing the death toll to 16, including the gunman.
June 2, 2010: Britain - Gunman Derrick Bird opens fire on people in towns across the rural county of Cumbria. Twelve people are killed and 11 injured. Bird also killed himself.
Jan 8, 2011: United States - Then-US congresswomen Gabrielle Giffords is target of an assassination attempt in Arizona in which six people are killed and 13 wounded. A person familiar with the case said this month that Jared Loughner, the man accused of the killings and wounding Giffords, is set to plead guilty in a Tucson court on Tuesday.
April 9, 2011: Netherlands - Tristan van der Vlis opened fire in the Ridderhof mall in Alphen aan den Rijn, south of Amsterdam, killing six before turning the gun on himself.
July 22, 2011: Norway - Police seize a gunman who killed 69 people at a youth summer camp of Norway's ruling political party, on the small, holiday island of Utoeya. Anders Behring Breivik is later charged with the killings, as well as with an earlier bombing in Oslo which killed eight people. The trial ended last month with Breivik saying that his bombing and shooting rampage was necessary to defend the country - prompting a walk-out by relatives of his victims.
Dec 13, 2011: Belgium - Gunman Nordine Armani kills three people, including a 17-month-old toddler, and wounds 121 in a central square in the eastern city of Liege, before shooting himself. The next day Belgian investigators find the body of a woman in warehouse used by the gunman raising the death toll, including the killer, to five.
Feb 27, 2012: United States - A student opens fire in a cafeteria at Chardon High School in Ohio, killing three students and injuring two others before being arrested.
April 2, 2012: United States - A gunman, identified by police as Korean-American One Goh, kills seven people and wounds three others in a shooting rampage at a Christian college in Oakland. The former nursing student pleaded not guilty to murder and attempted murder charges.
July 20, 2012: United States - A masked gunman kills 14 people and wounds 50 others when he opens fire on moviegoers at a showing of new Batman film "The Dark Knight Rises" in Aurora, a suburb of Denver, Colorado.
Aug. 5, 2012: United States - A gunman shoots six people dead during Sunday services at a Sikh temple in Oak Creek, Wisconsin, before he is shot dead by a police officer.
Sources: Reuters and Milwaukee Journal Sentinel