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

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