Here is a timeline of some of the worst shooting incidents in recent times carried out by one or two gunmen in the United States:
April 1999 - 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 - A gunman kills nine people at two brokerages in Atlanta, after apparently killing his wife and two children. He commits suicide five hours later.
October 2002 - John Muhammad and Lee Malvo kill 10 people in a string of sniper-style shootings that terrorize the Washington, D.C. area.
November 21, 2004 - A 35-year-old Hmong immigrant and naturalized U.S. citizen allegedly shoots eight people, killing six, while deer hunting east of Birchwood in northern Wisconsin.
April 16, 2007 - Virginia Tech, a university in Blacksburg, Virginia, becomes site of the deadliest rampage in U.S. history when a gunman kills 32 people and himself.
January 8, 2011 - Then-U.S. congresswomen Gabrielle Giffords is target of an assassination attempt in Tuscon, Arizona in which six people are killed and 13, including Giffords, wounded.
April 2, 2012 - 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.
July 20, 2012 - A masked gunman kills 12 people and wounds 58 others when he opens fire on moviegoers at a showing of the recent Batman film "The Dark Knight Rises" in Aurora, a suburb of Denver, Colorado.
August 5, 2012 - A gunman kills six people during Sunday services at a Sikh temple in Oak Creek, Wisconsin, before he is shot dead by a police officer.
August 24, 2012 - Two people are killed and eight wounded in a shooting outside the landmark Empire State Building in New York City at the height of the tourist season.
Sources: Reuters and Milwaukee Journal Sentinel (Reporting by David Cutler, London Editorial Reference Unit; Editing by Vicki Allen)