Below is a timeline of mass shootings in the United States over the past 20 years.
Feb. 7, 2008: Kirkwood, Mo. City Council shooting
Five people were killed when Charles Lee Thornton opened fire at Kirkwood City Hall. Thornton was then shot and killed by police.
Jan. 8, 2011: Political event shooting in Tucson, Ariz.
Six people were killed and 14 were injured, including Rep. Gabrielle Giffords, when Jared Loughner opened fire outside a supermarket in Tuscon, Ariz. Loughner was sentenced to life in prison without parole.
August 3, 2010: Connecticut warehouse shooting
Eight people were killed when Omar S. Thornton, a former employee of Hartford Distributors, opened fire in the beer distributor's warehouse. Thornton later turned the gun on himself.
Nov. 29, 2009: Lakewood, Wash. police officer shooting
Four police officers were killed when Maurice Clemmons entered a Parkland, Wash. coffee shop and opened fire. Clemmons was later killed by police after a two-day manhunt.
Dec. 5, 2007: Westroads Mall shooting
Eight people were killed and four injured when Robert A. Hawkins opened fire at the Omaha, Neb. mall before turning the gun on himself.
Apr. 16, 2007: Virginia Tech shooting
Thirty-two people were killed when student and shooter Seung-Hui Cho opened fire at the Virginia Tech campus in Blacksburg, Va. Cho then killed himself.
Nov. 2, 1999: Xerox murders
Seven people were killed when Bryan Uyesugi, an employee of the Xerox Engineering Systems office in Honolulu, Hawaii, opened fire on his co-workers.
Apr. 20, 1999: Columbine High School shooting
Twelve students and a teacher were killed when Eric Harris and Dylan Klebold opened fire at the Littleton, Colo. high school before killing themselves.
Apr. 3, 1995: Walter Rossler Company shooting
Five people were killed when Daniel Simpson opened fire on his former coworkers at Walter Rossler Company in Corpus Christi, Texas before taking his own life.
The Associated Press and Mother Jones contributed to this report.