A gunman shot a congresswoman in the head, seriously wounding her, and killed six other people in a shooting rampage at a public meeting in Tucson on Saturday.
The attack by a suspect authorities described as having a "troubled past" took place outside a supermarket where Gabrielle Giffords, a 40-year-old Democrat, was meeting with constituents.
Among the dead were a federal judge and a 9-year-old girl. Officials said 12 people were wounded.
The suspected gunman, identified by a federal law enforcement official as Jared Lee Loughner, 22, opened fire with a semi-automatic pistol at point-blank range. The suspect was tackled to the ground by two bystanders after the shooting and was in custody.
Giffords, beginning her third term in the House of Representatives, was in critical condition after surgery at Tucson University Medical Center and doctors said they were cautiously optimistic about her prospects for recovery.
The shooting shocked Washington, where Congress called off a key vote on healthcare reform next week, and a nation that went through acrimonious midterm elections in November. Some suggested the political vitriol might have played a role in the rare shooting of a federal lawmaker.
It was not known if the shooting was connected to any political stance, although Pima County Sheriff Clarence Dupnik said he believed that Giffords was the intended target of the shooting.
"(The suspect) has kind of a troubled past and we're not convinced that he acted alone," Dupnik told a news conference. Authorities were seeking a second man in connection with the shooting, he said.
Dupnik said the suspect had made threats to kill in the past but not against Giffords. "All I can tell you is that this person may have a mental issue," Dupnik said.
Dr. Steven Rayle, who helped restrain the gunman, told CNN he was dressed in a shabby manner but looked focused as he fired indiscriminately into the crowd.
President Barack Obama sent FBI Director Robert Mueller to Arizona to oversee the investigation, telling reporters, "We don't yet know what provoked this unspeakable act."
"The surgeons I spoke to are cautiously optimistic (that Giffords will survive)," Richard Carmona, a former US surgeon general and family friend, told the Tucson news conference. "With guarded optimism I hope she will survive."
House cancels votes
Giffords was hosting a "Congress on Your Corner" event -- public gatherings to give her constituents a chance to talk directly with her -- when the gunman attacked from about 4 feet away, National Public Radio said.
He approached Giffords from behind, firing at least 20 shots at her and others in the crowd, MSNBC said, citing law enforcement officials and witnesses.
The shooting prompted lawmakers in Washington to postpone their agenda for next week, including a vote on the repeal of Obama's healthcare overhaul. The new Congress convened this week after November 2 elections in which the Republican Party gained control of the House.
Giffords, a supporter of healthcare reforms that passed last year, had said that heated political rhetoric had prompted violent threats against her and vandalism at her office.
In an interview last year with the MSNBC television network, Giffords cited a map of electoral targets put out by former Alaska Republican Governor and prominent conservative Sarah Palin, each marked by the crosshairs of a rifle sight.
"When people do that, they've got to realize that there's consequences to that action," Giffords told MSNBC.
Palin quickly condemned the shootings on Saturday and offered condolences to the victims.
FBI investigates videos
In several videos on the Internet site YouTube, a person who posted under the name Jared Lee Loughner criticizes the government and religion and calls for a new currency. It was not known if he was the same person as the suspect.
"The government is implying mind control and brainwash on the people by controlling grammar. No! I won't pay debt with a currency that's not backed by gold and silver! No! I won't trust in God!" the site said.
The FBI was investigating whether the shooting suspect was the same person who posted the videos.
In a biographical sketch on the site, the author of the post writes that he attended Tucson-area schools and that his favorite books include Adolf Hitler's "Mein Kampf", Karl Marx's "Communist Manifesto," and Ken Kesey's "One Flew Over the Cuckoo's Nest," set in an insane asylum.
"My favorite activity is conscience dreaming: the greatest inspiration in my political business information," the writer says.
CNN quoted law enforcement authorities as saying the suspect's gun had been purchased legally. The US Army said in a statement released to the media that Loughner had tried unsuccessfully to enlist in the military.
Giffords, who is married to a NASA astronaut, is a rising star in the Democratic Party. She narrowly defeated a conservative opponent and was one of the few Democrats to survive the Republican sweep in swing districts in the November elections.
Her state has been at the center of a political firestorm the past year, symbolizing a bitter partisan divide across much of America.
The spark was the border state's move to crack down on illegal immigration last summer, a bill proposed by conservative lawmakers and signed by Republican Governor Jan Brewer.
Most Arizonans supported it, but opponents and many in the large Hispanic population felt it was unconstitutional and would lead to discrimination. Giffords said it would not secure the border or stop drug smuggling and gun running.
Dupnik, who was a friend of federal judge John Roll, one of those killed, criticized the political environment in Arizona and the rest of the country, and speculated it might have had a role in the shooting.
"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," Dupnik said.
"And, unfortunately, Arizona I think has become sort of the capital. We have become the Mecca for prejudice and bigotry."
House Speaker John Boehner, who holds the top post in the House of Representatives, said in statement he was horrified by the attack on Giffords and members of her staff. He called a news conference for 8:30 a.m. on Sunday.