Pima County Sheriff Clarence Dupnik said Giffords was the target of a gunman whom he described as mentally unstable and possibly acting along with an accomplice.
He said Giffords was among 13 people wounded in the melee that killed six people, including Arizona's chief federal judge, a 9-year-old girl and an aide for the Democratic lawmaker. He said the rampage ended only after two people tackled the gunman.
Doctors were optimistic about Giffords surviving as she was responding to commands from doctors despite having a bullet go through her head.
"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.
The sheriff pointed to the vitriolic political rhetoric that has consumed the country as he denounced the shooting that claimed several of his friends as victims, including U.S. District Judge John Roll. The judge celebrated Mass on Saturday morning like he does every day before stopping by to say hello to his good friend Giffords.
"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," the sheriff said. "And unfortunately, Arizona I think has become the capital. We have become the mecca for prejudice and bigotry."
The reaction to the shooting rippled across the country as Americans were aghast at the sight of such a violent attack on a sitting member of Congress. The shooting cast a pall over the Capitol as politicians of all stripes denounced the shooting as a horrific and senseless act of violence.
Obama dispatched his FBI director to Arizona. Capitol police asked members of Congress to be more vigilant about security in the wake of the shooting, and some politicians expressed hope that the killing spree serves as a wakeup call at a time when the political climate has become so emotionally charged.
"It is a tragedy for Arizona, and a tragedy for our entire country," President Barack Obama declared.
Giffords, 40, is a three-term moderate Democrat who narrowly won re-election in November against a tea party candidate as conservatives across the country sought to throw her from office over her support of the health care law. Her office in Tucson was vandalized in the hours after the House passed the overhaul last March as anger over the law spread across the country.
Jared L. Loughner (AP Photo/Arizona Daily Star, Mamta Popat)
Police say the shooter was in custody, and was identified by people familiar with the investigation as Jared Loughner, 22. Officials said he used a pistol with an extended clip to carry out the attack. U.S. officials who provided his name to the AP spoke on condition of anonymity because they were not authorized to release it publicly.
The suspect's exact motivation was not clear, but a former classmate described Loughner as a pot-smoking loner who had rambling beliefs about the world. The Army said he tried to enlist in December 2008 but was rejected for reasons the military did not provide.
Federal law enforcement officials were poring over versions of a MySpace page that belonged to Jared Loughner and over a YouTube video published weeks ago under an account "Classitup10" and linked to him. The MySpace page, which was removed within minutes of the gunman being identified by officials, 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)."
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 as a judge.
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.
Giffords 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 her husband 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, 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."
Giffords has drawn the ire of the right in the last year, especially from politicians like Sarah Palin over her support of the health care bill. It's still not clear if the gunman had the health care debate in mind or was focused on his own unique set of political beliefs as witnessed in the Internet videos.
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.
Giffords' Tucson office was vandalized a few hours after the House voted to approve the health care law in March, with someone either kicking or shooting out a glass door and window. More recently, the sheriff also said that someone in a "very angry audience" at a Giffords event dropped a weapon out of their pants.
In an interview after the vandalism, Giffords referred to the animosity against her by conservatives. Palin listed Giffords' seat as one of the top "targets" in the midterm elections because of the lawmakers' support for the health care law.
"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.
The sheriff used the word "vitriol" several times to express his disgust with the political climate in the country and in Arizona, a heavily conservative state that put itself at the center of the national debate on immigration last year with a contentious crackdown on illegal immigrants. Residents and politicians here have also vocally opposed Obama and Democrats on health care.
The shooting occurred at a shopping center called La Toscana Village as Giffords met with voters outside a Safeway grocery store.
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.
Law enforcement officials and reporters from around the country quickly descended on Tucson, the second biggest city in the state and home to the University of Arizona. The scene has been converted into a command post with about a dozen or so emergency vehicles and agents in FBI jackets milling about the location.
Outside Giffords' office on Capitol Hill, a handful of congressional staffers could be seen walking into her office without comment, some with roller bags and one who was in tears. About a half dozen yellow flowers placed by one mourner sat outside the door.
In Loughner's middle-class neighborhood - about a five-minute drive from the scene - sheriff's deputies had much of the street blocked off as curious neighbors asked what was going on. The neighborhood sits just off a bustling Tucson street and is lined with desert landscaping and palm trees.
Neighbors said Loughner kept to himself but that they often saw him walking his dog, almost always wearing a hooded sweat shirt listening to his iPod. Neighbors said Loughner lived with his parents.
Loughner's MySpace profile indicates he attended and graduated from school in Tucscon 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.
The shooting comes amid a highly charged political environment that has seen several dangerous threats against lawmakers but nothing that reached the point of actual violence.
A San Francisco man upset with House Speaker Nancy Pelosi's support of health care reform pleaded guilty to threatening the Democratic congresswoman and her family, calling her directly on March 25 and threatening to destroy her Northern California home if she voted for health care reform.
In July, a California man known for his anger over left-leaning politics engaged in a shootout with highway patrol officers after planning an attack on the ACLU and another nonprofit group. The man said he wanted to "start a revolution" by killing people at the ACLU and the Tides Foundation.
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," Ellinwood said.
Giffords is known in her southern Arizona district for her numerous public outreach meetings, which she admitted 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."
Associated Press Writers 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.