A History of Violence in America

DECEMBER 14, 2012: Connecticut State Police lead children from the Sandy Hook Elementary School in Newtown, Conn., following a shooting. At least 20 people were killed, including many children. (AP Photo/Newtown Bee, Shannon Hicks)

AUGUST 5, 2012: A man sits on a rock as police investigate the Sikh Temple of Wisconsin in Oak Creek, Wis., after a shooting.   (AP Photo/Jeffrey Phelps)

JULY 20, 2012: Police respond to a theater in Aurora, Colo., on July 20, 2012, where a gunman killed at least 12 people and injured dozens during a showing of the new Batman movie.   (AP Photo/Ed Andrieski)

JAN. 8, 2011: A lone bouquet of flowers lies on the floor near the Capitol Hill office of Rep. Gabrielle Giffords, D-Ariz. Giffords survived after being shot point-blank in the head when a gunman opened fire at a political event in Tucson on Jan. 8, 2011. (AP Photo/Susan Walsh)

NOV. 5, 2009: The casket of Pfc. Kham Xiong is carried to his burial site at Fort Snelling National Cemetery in Minneapolis on Nov. 30, 2009. Xiong was killed in the Nov. 5 shooting at Ft. Hood, Texas. Thirten people were killed and 30 wounded in the shooting by a lone gunman.   (AP Photo/Andy King)

APRIL 18, 2007: Students visit a makeshift memorial set up on the campus of Virginia Tech for the students and faculty that lost their lives in the deadliest mass shooting in U.S. history on April 18, 2007.   (AP Photo/Evan Vucci)

APRIL 22, 1999: Columbine High School students pay their respects at a makeshift memorial set up in a park near the high school in Colorado on April 22, 1999. Two students killed 13 people and then themselves in a shooting spree at the school. (AP Photo/Eric Gay)