Don Spirit is pictured in this 2001 handout photo obtained by Reuters on Friday. Police identified Spirit as the man who gunned down his six grandchildren and daughter before taking his own life. Reuters/Landov hide caption

itoggle caption Reuters/Landov

Police officers take cover behind their vehicles in Moncton, New Brunswick, on Wednesday. Three police officers were shot dead and two others were injured in a shooting Wednesday night; Canadian police are searching for the suspected gunman. Ron Ward via the Canadian Press/AP/Moncton Times & Transcript hide caption

itoggle caption Ron Ward via the Canadian Press/AP/Moncton Times & Transcript

Richard Martinez, whose son Christopher Michael-Martinez was killed in Friday's mass shooting in Isla Vista, Calif., breaks down as he talks to media outside the Santa Barbara County Sheriff's headquarters on Saturday. Jae C. Hong/AP hide caption

itoggle caption Jae C. Hong/AP

Newtown, Conn., Dec. 20, 2012: Stuffed animals and a candle arrangement at a streetside memorial for the 20 children and six adults killed at Sandy Hook Elementary School. John Moore/Getty Images hide caption

itoggle caption John Moore/Getty Images

Former Arizona Rep. Gabrielle Giffords, accompanied by her husband, retired astronaut Mark Kelly, speaks during a news conference in Manchester, N.H., in July. They were there to encourage state political leaders to have courage in the fight to expand background checks on gun purchases. Mary Schwalm/AP hide caption

itoggle caption Mary Schwalm/AP

President Obama speaks during a memorial service at the Marine Barracks Sunday in Washington, D.C., honoring victims of Monday's shooting at the Navy Yard. Brendan Smialowski/AFP/Getty Images hide caption

itoggle caption Brendan Smialowski/AFP/Getty Images

The scene late Thursday at a park in Chicago's South Side after a shooting there in which 13 people were wounded. Paul Beaty /AP hide caption

itoggle caption Paul Beaty /AP

Police say Aaron Alexis, who was killed Monday, gunned down 12 people at the Washington Navy Yard. FBI/Reuters/Landov hide caption

itoggle caption FBI/Reuters/Landov

At the White House and around the nation, flags are flying at half-staff since Monday's mass shooting at the Washington Navy Yard. Olivier Douliery /UPI/Landov hide caption

itoggle caption Olivier Douliery /UPI/Landov

Aaron Alexis, whom the FBI believes to have been responsible for the shootings at the Washington Navy Yard in Washington, D.C., is shown in this handout photo released by the FBI on Monday. FBI/Reuters/Landov hide caption

itoggle caption FBI/Reuters/Landov

In Herkimer, N.Y., police and suspect Kurt Myers were in a standoff from midday Wednesday into early early Thursday. Overnight, these officers were on the town's North Main Street. Brett Carlsen/Getty Images hide caption

itoggle caption Brett Carlsen/Getty Images