|Giffords, who is married to Mark Kelly, a NASA astronaut, represents Arizona's eighth congressional district [AFP]
US congresswoman Gabrielle Giffords is said to be responding well to treatment after an assailant shot her in the head and killed six others in a rampage in Tucson, Arizona.
The 40-year-old Democrat was "able to communicate" by following simple commands, but had not yet spoken, one of her trauma surgeons said on Sunday.
Noting that the bullet did not cross through the centre of Giffords' brain, Michael Lemole, chief of neurosurgery at University Medical Centre, said: "[She] is able to communicate with us this morning through following simple commands.
"We're very encouraged by that. We are still in critical condition. Brain swelling at any time can take a turn for the worse. But I am cautiously optimistic."
On Sunday federal prosecutors in Tucson filed five charges, including murder and attempted murder, against Jared Loughner, 22, over the attack.
Investigators said they had carried out a search warrant at the suspect's home and seized an envelope from a safe with messages such as "I planned ahead", "My assassination" and the name "Giffords" next to what appears to be the man's signature.
Saturday's shooting of the politician, who represents Arizona's eighth congressional district in the House, has shocked politicians in Washington, where congress postponed a vote on healthcare reform later this week.
Arizona authorities say a second man has been cleared of any involvement in the attack.
Jason Ogan, Pima County sheriff's deputy, said the man "came by, and we talked to him and we determined he was not involved".
He also said authorities were not ready to say if they are confident the armed man acted alone.
Following an acrimonious campaign in the run-up to last November's midterm congressional elections, some commentators were quick to cite a shrill climate of political vitriol might have played a role in the shooting.
"We are in a dark place in this country right now and the atmospheric condition is toxic," Democratic representative Emanuel Cleaver told NBC's Meet the Press.
The suspected attacker opened fire with a semi-automatic pistol at point-blank range outside a supermarket.
Six people, including a US federal judge and a nine-year-old girl, were killed in the attack, with Giffords and 13 others wounded.
Police seeking a motive for the shooting spree were looking at a trail of anti-government messages on the internet left either by Loughner or someone writing under that name.
There was no coherent theme to the messages.
John Boehner, the US House of Representatives speaker, ordered flags at the US Capitol in Washington lowered to half staff in memory of the victims.
Boehner said the incident was a reminder that public service comes with a risk.
"This inhuman act should not and will not deter us from our calling to represent our constituents and fulfill our oaths of office. No act, no matter how heinous, must be allowed to stop us from our duty," Boehner said.
In Tucson, Clarence Dupnik, the Pima County sheriff, said the suspect "has kind of a troubled past and we're not convinced that he acted alone".
He said he believed Giffords was the intended target of the shooting.
Dupnik said the suspect had made threats to kill in the past but not against Giffords.
Barack Obama, the US president, has put Robert Mueller, the FBI director, in charge of the investigation, with a team heading to the scene of the murders.