The following are the main mass shootings that have occurred in the United States since 1999.

-- In April 1999, two teenage schoolboys shot and killed 12 schoolmates and a teacher at Columbine High School in Littleton, Colorado, before killing themselves.

-- In July 1999, a stock exchange trader in Atlanta, Georgia, killed 12 people including his wife and two children before taking his own life.

-- In September 1999, a gunman opened fire at a prayer service in Fort Worth, Texas, killing six people before committing suicide.

-- In October 2002, a series of sniper-style shootings occurred in Washington DC, leaving 10 dead.

-- In August 2003 in Chicago, a laid-off worker shot and killed six of his former workmates.

-- In November 2004 in Birchwood, Wisconsin, a hunter killed six other hunters and wounded two others after an argument with them.

-- In March 2005, a man opened fire at a church service in Brookfield, Wisconsin, killing seven people.

-- In October 2006, a truck driver killed five schoolgirls and seriously wounded six others in a school in Nickel Mines, Pennsylvania before taking his own life.

-- In April 2007, a student shot and killed 32 people and wounded 15 others at Virginia Tech in Blacksburg, Virginia, before shooting himself, making it the deadliest mass shooting in the United States after 2000.

-- In December 2007, a 20-year-old man killed nine people and injured five others in a shopping center in Omaha, Nebraska.

-- In December 2007, a woman and her boyfriend shot dead six members of her family on Christmas Eve in Carnation, Washington.

-- In February 2008, a shooter who is still at large tied up and shot six women at a suburban clothing store in Chicago, leaving five of them dead and the remaining one injured.

-- In February 2008, a man opened fire in a lecture hall at Northern Illinois University in DeKalb, Illinois, killing five students and wounding 16 others before laying down his weapon and surrendering.

-- In September 2008, a mentally ill man who was released from jail one month earlier shot eight people in Alger, Washington, leaving six of them dead and the rest two wounded.

-- In December 2008, a man dressed in a Santa Claus suit opened fire at a family Christmas party in Covina, California, then set fire on the house and killed himself. Police later found nine people dead in the debris of the house.

-- In March 2009, a 28-year-old laid-off worker opened fire while driving a car through several towns in Alabama, killing 10 people.

-- In March 2009, a heavily-armed gunman shot dead eight people, many of them elderly and sick people, in a private-owned nursing home in North Carolina.

-- In March 2009, six people were shot dead in a high-grade apartment building in Santa Clara, California.

-- In April 2009, a man shot dead 13 people at a civic center in Binghamton, New York.

-- In November 2009, U.S. army psychologist Major Nidal Hasan opened fire at a military base in Fort Hood, Texas, leaving 13 dead and 42 others wounded.

-- In January 2011, a gunman opened fire at a public gathering outside a grocery in Tuscon, Arizona, killing six people including a nine-year-old girl and wounding at least 12 others. Congresswoman Gabrielle Giffords was severely injured with a gunshot to the head.