Democratic Representative Gabrielle Giffords, 40, was shot at close range during a public meeting in Tucson.

She is in a critical condition, but the doctor treating her said he was "very optimistic about her recovery".

The dead included a nine-year-old girl and a federal judge. President Barack Obama said the shooting was a "tragedy for our entire country".

The Associated Press news agency reported that one of Ms Giffords' political aides was also killed in the shooting.

Police said a total of 13 people were shot and wounded during the attack. Doctors said five of them including Ms Giffords were in a critical condition.

A suspect named in local media as 22-year-old Arizona resident Jared Loughner was arrested after the incident.

Local police, who have not confirmed the suspect's name, said they were not convinced that the gunman was working alone.

"There is some reason to believe that he came to this location with another individual," said Pima County Sheriff Clarence Dupnik.

He said police were trying to trace the other person, who he said might have been involved.