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A shaken President Obama called the attack “a tragedy for our entire country.”

Attorney General Eric H. Holder Jr. said in a statement Sunday that FBI Director Robert S. Mueller III was traveling to Arizona to help coordinate the investigation.

In a brief statement Sunday morning, House Speaker John A. Boehner, Ohio Democrat, said flags on the House side of the Capitol in Washington will be flown at half staff to honor Ms. Giffords‘ slain aide, Gabe Zimmerman. Mr. Boehner said normal House business this week is postponed to focus on any necessary actions in the shooting aftermath.

Strong reaction came from overseas, as well.

British Prime Minister David Cameron expressed shock at the shooting and added that he shared Mr. Obama’s belief that “we must never allow violence and hate to extinguish the open political discourse which is our surest protection.”

Fidel Castro also denounced the attack as atrocious. “Even those of us who don’t share at all the politics and philosophies [of the Obama administration] sincerely desire that no children, judges, legislators or citizens of the United States die in such an absurd and unjustifiable way,” Mr. Castro said in an opinion piece titled “An Atrocious Act,” published in Cuban state-controlled media.

Ms. Giffords is a moderate Democrat who narrowly won re-election in November against a tea party candidate who sought to throw her from office over her support of the health care law. Anger over her position became violent at times, with her Tucson office vandalized after the House passed the overhaul last March and someone showing up at a recent gathering with a weapon.

Authorities said the dead included U.S. District Judge John Roll, 63; Christina Greene, 9; Mr. Zimmerman, 30; Dorothy Morris, 76; Dorwin Stoddard, 76; and Phyllis Scheck, 79.

Dupnik lashed out at what he called an excessively “vitriolic” atmosphere in the months leading up to the rampage as he described the chaos of the day.

Ms. Giffords expressed similar concern about the political atmosphere, even before the shooting. In an interview after her office was vandalized, she referred to the animosity against her by conservatives, including Sarah Palin’s decision to list Ms. Giffords‘ seat as one of the top “targets” in the midterm elections.

“For example, we’re on Sarah Palin’s targeted list, but the thing is, that the way that she has it depicted has the crosshairs of a gun sight over our district. When people do that, they have to realize that there are consequences to that action,” Ms. Giffords said in an interview with MSNBC.

In the hours after the shooting, Mrs. Palin issued a statement in which she expressed her “sincere condolences” to the family of Ms. Giffords and the other victims.

During his campaign effort to unseat Ms. Giffords in November, Republican challenger Jesse Kelly held fundraisers where he urged supporters to help remove Ms. Giffords from office by joining him to shoot a fully loaded M-16 rifle. Mr. Kelly is a former Marine who served in Iraq and was pictured on his website in military gear holding his automatic weapon and promoting the event.

“I don’t see the connection” between the fundraisers featuring weapons and Saturday’s shooting, said John Ellinwood, Mr. Kelly’s spokesman. “I don’t know this person; we cannot find any records that he was associated with the campaign in any way. I just don’t see the connection.

“Arizona is a state where people are firearms owners — this was just a deranged individual.”

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