Authorities said Gifford, 40, was targeted at a public gathering by a man with a semiautomatic weapon around 10 a.m. Saturday outside a busy Tucson supermarket. Arizona's chief federal judge and five others were killed and 14 people were wounded, including the Democrat lawmaker.
He also fired at her district director and shot indiscriminately at staffers and others standing in line to talk to the congresswoman, said Mark Kimble, a communications staffer for Giffords.
"He was not more than three or four feet from the congresswoman and the district director," Kimble said, describing the scene as "just complete chaos, people screaming, crying."
Pima County Sheriff Clarence Dupnik said the rampage ended only after two people tackled the gunman.
"He was definitely on a mission," said Villec, a former Giffords intern.
Video: Suspect: 'You could call me a terrorist' (on this page)Police say the shooter was in custody, and was identified by people familiar with the investigation as Jared Loughner, 22. U.S. officials who provided his name to the AP spoke on condition of anonymity because they were not authorized to release it publicly.
His motivation was not immediately known, but Dupnik described him as mentally unstable and possibly acting with an accomplice.
His office said a man possibly associated with the suspect who was near the scene was being sought. The man, who was photographed by a security camera, was described as white with dark hair and 40-45 years old.
The assassination attempt left Americans questioning whether divisive politics had pushed the suspect over the edge.
A shaken President Barack Obama called the attack "a tragedy for our entire country."
"We are in a dark place in this country right now and the atmospheric condition is toxic," Democratic Representative Emanuel Cleaver told NBC's "Meet the Press."
Attorney General Eric Holder said in a statement Sunday that FBI Director Robert Mueller was traveling to Arizona to help coordinate the investigation.
In a brief statement Sunday morning, House Speaker John Boehner said flags on the House side of the Capitol in Washington will be flown at half staff to honor Giffords' slain aide, Gabe Zimmerman. Boehner says 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 President Barack 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," Castro said in an opinion piece titled "An Atrocious Act," published in Cuban state-controlled media.
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; Giffords aide Zimmerman, 30; Dorothy Morris, 76; Dorwin Stoddard, 76; and Phyllis Scheck, 79.
The sheriff blamed the vitriolic political rhetoric that has consumed the country, much of it occurring in Arizona.
Giffords expressed similar concern, 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 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," Giffords said in an interview with MSNBC.
In the hours after the shooting, Palin issued a statement in which she expressed her "sincere condolences" to the family of Giffords and the other victims.
Story: Profile of suspect Jared Loughner: ‘I can't trust the current government’
During his campaign effort to unseat Giffords in November, Republican challenger Jesse Kelly held fundraisers where he urged supporters to help remove Giffords from office by joining him to shoot a fully loaded M-16 rifle. 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, 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."
Law enforcement officials said members of Congress reported 42 cases of threats or violence in the first three months of 2010, nearly three times the 15 cases reported during the same period a year earlier. Nearly all dealt with the health care bill, and Giffords was among the targets.
The shooting cast a pall over the Capitol as politicians of all stripes denounced the attack as a horrific. Capitol police asked members of Congress to be more vigilant about security in the wake of the shooting. Obama dispatched his FBI chief to Arizona.
Slaten said nine other wounded were being treated at the hospital, four of them critical and five of them serious. Slaten said the three others were treated at other hospitals and released.
Giffords spokesman C.J. Karamargin said besides the aide Zimmerman, who was killed, two other Giffords staffers were shot but expected to survive. Zimmerman was a former social worker who served as Giffords' director of community outreach.
The 9-year-old girl who died was the granddaughter of former Phidadelphia Phillies manager Dallas Green, who is now an executive advisor for the team he managed to the World Series championship in 1980.
"She was a talented young girl with a bright promising future. Her untimely death weighs heavily on our hearts," team president David Montgomery said Sunday. The Phillies said the girl's name was spelled Christina-Taylor Green; Arizona authorities provided a different spelling, Christina Greene.
Greg Segalini, an uncle of the girl, told the Arizona Republic that a neighbor was going to the event and invited her along because she had just been elected to the student council and was interested in government.
Christina, who was born on Sept. 11, 2001, was involved in various activities, from ballet to baseball, and had just received her first Holy Communion at St. Odilia's Catholic Church in Tucson, Catholic Diocese of Tucson officials told The Arizona Daily Star.
Her birth date no doubt helped prod the girl's interest in politics, her mother, Roxanna Green, told the Star. She was one of 50 babies born on Sept. 11 featured in a book called "Faces of Hope."
"She was born back East and Sept. 11 affected everyone there, and Christina-Taylor was always very aware of it. She was very patriotic and wearing red, white and blue was really special to her," her mother said.
The suspect Loughner was described by a former classmate as a pot-smoking loner, and the Army said he tried to enlist in December 2008 but was rejected for reasons not disclosed.
Federal law enforcement officials were poring over versions of a MySpace page that included a mysterious "Goodbye friends" message published hours before the shooting and exhorted his friends to "Please don't be mad at me."In one of several YouTube videos, which featured text against a dark background, Loughner described inventing a new U.S. currency and complained about the illiteracy rate among people living in Giffords' congressional district in Arizona.
"I know who's listening: Government Officials, and the People," Loughner wrote. "Nearly all the people, who don't know this accurate information of a new currency, aren't aware of mind control and brainwash methods. If I have my civil rights, then this message wouldn't have happen (sic)."
Video: An outpouring of grief in stricken Tucson (on this page)In Loughner's middle-class neighborhood — about a five-minute drive from the scene — sheriff's deputies had much of the street blocked off. The neighborhood sits just off a bustling Tucson street and is lined with desert landscaping and palm trees.
Neighbors said Loughner lived with his parents and kept to himself. He was often seen walking his dog, almost always wearing a hooded sweat shirt and listening to his iPod.
Loughner's MySpace profile indicates he attended and graduated from school in Tucson and had taken college classes. He did not say if he was employed.
High school classmate Grant Wiens, 22, said Loughner seemed to be "floating through life" and "doing his own thing."
"Sometimes religion was brought up or drugs. He smoked pot, I don't know how regularly. And he wasn't too keen on religion, from what I could tell," Wiens said.
Lynda Sorenson said she took a math class with Loughner last summer at Pima Community College's Northwest campus and told the Arizona Daily Star he was "obviously very disturbed." "He disrupted class frequently with nonsensical outbursts," she said.
In October 2007, Loughner was cited in Pima County for possession of drug paraphernalia, which was dismissed after he completed a diversion program, according to online recordsGiffords was first elected to Congress amid a wave of Democratic victories in the 2006 election, and has been mentioned as a possible Senate candidate in 2012 and a gubernatorial prospect in 2014.
She is married to astronaut Mark E. Kelly, who has piloted space shuttles Endeavour and Discovery. The two met in China in 2003 while they were serving on a committee there, and were married in January 2007. Sen. Bill Nelson, chairman of the Senate Commerce Space and Science Subcommittee, said Kelly is training to be the next commander of the space shuttle mission slated for April. His brother is currently serving aboard the International Space Station, Nelson said.
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Associated Press Writers Amanda Lee Myers and Terry Tang in Tucson, Jacques Billeaud, Bob Christie and Paul Davenport in Phoenix, and David Espo, Matt Apuzzo, Eileen Sullivan, Adam Goldman and Charles Babington in Washington contributed to this report.