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

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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

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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

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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

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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

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

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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

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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

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