TUCSON, Ariz. » A gunman fired a semiautomatic weapon at a busy supermarket yesterday 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 Arizona 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 oust her over her support for 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 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 toxic 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 cross hairs 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.

Giffords was among 20 House Democratic supporters of the health care bill who were the subject of a posting on the former Alaska governor’s Facebook page encouraging people to organize against their re-election. 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.

Yesterday’s shooting cast a pall over Washington as politicians of all stripes denounced the attack. 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.

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.

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, a former U.S. 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.

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.

In Loughner’s middle-class neighborhood — about a five-minute drive from the scene — sheriff’s deputies had much of the street blocked off.

Neighbors said Loughner lived with his parents and kept to himself.

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.