Doctors were cautiously optimistic about the condition of US congresswoman Gabrielle Giffords after a man shot her in the head and killed six people at a public event in Arizona.

Jared Lee Loughner, 22, the man suspected of killing six people and wounding a US congresswoman in Arizona, has been charged with five criminal counts including attempted assassination.

The charges were filed in US federal court in Tucson.

Mr Loughner faces one count of attempted assassination of a member of Congress; two counts of killing an officer of the United States; and two counts of attempted killing of an officer of the United States.

‘It appears that the target was the congresswoman,’ FBI Director Robert Mueller told a news conference and added that More charges against Mr Loughner are possible.

Public officials should be on alert but there was no information to suggest a specific threat, he said.

Ms Giffords, a 40-year-old Democrat, is in critical condition but is able to follow simple commands, such as holding up two fingers when asked, doctors at University Medical Center in Tucson said.

A single bullet traveled the length of her brain on the left side, hitting an area that controls speech functions.

Given the devastating wound, doctors said they were uncertain about the extent of brain damage she had suffered.

Ms Giffords has been put into a medical coma but was being woken frequently to check her progress.

The suspect opened fire with a semi-automatic pistol at point-blank range outside a supermarket, killing six people including US federal judge John Roll and a 9-year-old girl. Fourteen people were wounded.

Pima County Sheriff Clarence Dupnik said a wounded woman clawed away an ammunition magazine from the gunman, possibly preventing even more people from being shot.

The gunman managed to reload his Glock pistol but the fresh magazine would not work and the suspect was then tackled by two men.

Arizona police released a photo of another man sought for questioning who was seen at the shopping centre, but he was not believed to be directly involved in the shooting.

The attack shocked Washington, where Congress postponed a key vote on healthcare reform later this week.

It was not known if the shooting was connected to any political stance.

Pima County Sheriff Clarence Dupnik said the suspect 'has kind of a troubled past and we're not convinced that he acted alone'.

He said the suspect had made threats to kill in the past but not against Ms Giffords.

'All I can tell you is that this person may have a mental issue,' he added, describing him as unstable.

Dr Steven Rayle, who helped restrain the suspect, told CNN the gunman was dressed in a shabby manner but looked focused as he fired indiscriminately into the crowd.

President Obama said the attack was a tragedy for the whole country. Arizona's Republican Senator John McCann also voiced his shock at the shootings.

Ms 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 1.2m away, according to media reports.

Palin removes map of electoral targets

Ms Giffords had warned previously that the heated political rhetoric had prompted violent threats against her and vandalism at her office.

In an interview last year, she cited a map of electoral targets put out by prominent conservative Sarah Palin, each marked by the cross hairs of a rifle sight.

'When people do that, they've got to realide that there's consequences to that action,' Ms Giffords told MSNBC.

Ms Palin removed the graphic from her website yesterday and offered her condolences on a posting on Facebook.

US House of Representatives Speaker John Boehner has ordered flags at the US Capitol in Washington lowered to half staff in memory of the victims.

'This inhuman act should not and will not deter us from our calling to represent our constituents and fulfil our oaths of office. No act, no matter how heinous, must be allowed to stop us from our duty,' Mr Boehner said.