Here is a timeline of some of the worst mass shootings in the United States over the past 25 years:

- Killeen, Texas, October 16, 1991: A man shoots dead 23 people in a restaurant and then kills himself.

- Littleton, Colorado, April 20, 1999: Two teenage boys shoot and kill 13 fellow classmates and a teacher at Columbine High School before killing themselves.

- Atlanta, Georgia, July 29, 1999: A stock market trader goes on a day-long shooting rampage, killing 12 people, including his wife and two children, before taking his own life.

- Red Lake, Minnesota, March 21, 2005: A teenage boy kills two people at his grandfather's home on an Indian reservation and then goes to his local high school where he kills seven others before committing suicide.

- Blacksburg, Virginia, April 16, 2007: A student goes on a rampage at Virginia Tech University, killing 32 people before committing suicide.

- Covina, California, December 24, 2008: A man dressed in a Santa Claus suit opens fire at a family party and then sets fire to the house. Nine people are killed. The gunman commits suicide.

- Binghamton, New York, April 3, 2009. A gunman shoots dead 13 people at a civic centre for immigrants.

- Fort Hood, Texas, November 5, 2009. US army psychiatrist Major Nidal Hasan opens fire at his military base, killing 13 people and wounding 42, before being overcome by police.

- Oikos, California, April 2, 2012: A male nursing student methodically kills seven people at a Christian university.

- Aurora, Colorado, July 20, 2012: A man kills 12 people when he opens fire at a movie theatre showing a late-night premiere of a Batman film in a suburb of Denver.

- Oak Creek, Wisconsin, August 5, 2012: A white supremacist and US army veteran fatally shoots six people and wounds four others at a Sikh temple before taking his own life.

- Newtown, Connecticut, December 14, 2012: A young man kills 26 people, including 20 children at Sandy Hook elementary school. He also fatally shoots his mother. He commits suicide.

- Miami, Florida, July 26, 2013: A gunman kills six people in a shooting rampage at an apartment building, taking two people hostage before being killed by police after a lengthy standoff.

- Washington, DC, September 16, 2013: A gunman fatally shoots 12 people at a naval base in the US capital. The gunman is killed by police.

- Charleston, South Carolina, June 17, 2015: A white gunman kills nine people at an historic black church in Charleston, South Carolina.

- Roseburg, Oregon, October 1, 2015: A gunman goes on a shooting rampage at a college in the western state of Oregon, killing 10 people and wounding seven before he died in a shoot-out with police.