United States officials have charged Jared Lee Loughner, the man suspected of killing six people and wounding a US congresswoman in Arizona, with five criminal counts including attempted assassination.

Congresswoman Gabrielle Giffords, who was shot in the head, is fighting for her life in hospital. A Federal Judge and a nine-year old girl are among the dead.

The 40-year-old Democratic lawmaker is in a critical condition and doctors are cautiously optimistic she will survive.

The suspect, Jared Lee Loughner, 22, allegedly opened fire with a semi-automatic pistol at point-blank range.

Police are looking for a possible accomplice. Pima County, Arizona Sheriff Clarence Dupnik said earlier on Sunday: "There's some reason to believe that he came to this location with another individual."