A gunman shot a US 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 US 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.'    

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 (1.2 metres) 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 Nov. 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. - Reuters