TUCSON, Ariz. (AP) — A gunman nearly unloaded a semiautomatic weapon at a busy supermarket Saturday during a public gathering for Rep. Gabrielle Giffords, wounding the Democrat and killing Arizona's chief federal judge and five others in an attempted assassination that left Americans questioning whether divisive politics had pushed the suspect over the edge.

The shooting targeted Giffords and left the three-term congresswoman in critical condition after a bullet passed through her head. A shaken President Barack Obama called the attack "a tragedy for our entire country."

Giffords, 40, 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.

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 Pima County Sheriff Clarence Dupnik described him as mentally unstable and possibly acting with an accomplice. He said Giffords was among 13 people wounded in the melee that killed six people, including a 9-year-old girl, an aide for the Democratic lawmaker and U.S. District Judge John Roll, who had just stopped by to see his friend Giffords after attending Mass. Dupnik said the rampage ended only after two people tackled the gunman.

The sheriff blamed the vitriolic political rhetoric that has consumed the country, much of it occurring in Arizona.

"When you look at unbalanced people, how they respond to the vitriol that comes out of certain mouths about tearing down the government. The anger, the hatred, the bigotry that goes on in this country is getting to be outrageous," he said. "And unfortunately, Arizona, I think, has become the capital. We have become the mecca for prejudice and bigotry."

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.

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.

Giffords, known as "Gabby," tweeted shortly before the shooting, describing her "Congress on Your Corner" event: "My 1st Congress on Your Corner starts now. Please stop by to let me know what is on your mind or tweet me later."

"It's not surprising that today Gabby was doing what she always does, listening to the hopes and concerns of her neighbors," Obama said. "That is the essence of what our democracy is about."

Mark Kimball, a communications staffer for Giffords, described the scene as "just complete chaos, people screaming, crying." The gunman fired at Giffords and her district director and started shooting indiscriminately at staffers and others standing in line to talk to the congresswoman, Kimball said.

"He was not more than three or four feet from the congresswoman and the district director," he said.

Doctors were optimistic about Giffords surviving as she was responding to commands from doctors. "With guarded optimism, I hope she will survive, but this is a very devastating wound," said Dr. Richard Carmona, the former surgeon general who lives in Tucson.

Giffords spokesman C.J. Karamargin said three Giffords staffers were shot. One died, and the other two are expected to survive. Gabe Zimmerman, a former social worker who served as Giffords' director of community outreach, died. Giffords had worked with the judge in the past to line up funding to build a new courthouse in Yuma, and Obama hailed him for his nearly 40 years of service.

An uncle of the 9-year-old 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.

A former classmate described Loughner 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)."

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.

"We're getting out of here. We are freaked out," 33-year-old David Cleveland, who lives a few doors down from Loughner's house, told The Associated Press.

Cleveland said he was taking his wife and children, ages 5 and 7, to her parent's home when they heard about the shooting.

"When we heard about it, we just got sick to our stomachs," Cleveland said. "We just wanted to hold our kids tight."

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 records.

"He has kind of a troubled past, I can tell you that," Dupnik said.

Giffords 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.

Giffords is known in her southern Arizona district for her numerous public outreach meetings, which she acknowledged in an October interview with The Associated Press can sometimes be challenging.

"You know, the crazies on all sides, the people who come out, the planet earth people," she said with a following an appearance with Adm. Mike Mullen in which the chairman of the Joint Chiefs of Staff was peppered with bizarre questions from an audience member. "I'm glad this just doesn't happen to me."

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Associated Press Writers Terry Tang and Pauline Arrillaga in Tucson, Jacques Billeaud, Bob Christie and Paul Davenport in Phoenix, and Espo, Matt Apuzzo, Eileen Sullivan, Adam Goldman and Charles Babington in Washington contributed to this report.