Mr Loughner, who is due to appear in court, could face the death penalty.

Ms Giffords is in a critical condition after being shot in the head at a public meeting in Tucson, Arizona.

She had been holding an open-invitation meeting with constituents outside a supermarket when a man holding a gun approached and opened fire.

She was shot from close range by the gunman, who then began shooting into the crowd.

Among the dead were a nine-year-old girl and a federal judge. A total of 14 people were injured, in addition to the six who were killed.

Local Sheriff Clarence Dupnik said a woman tackled（处理，抓住） the gunman as he tried to reload, snatching（抢夺） a magazine of bullets.

He managed to reload with another magazine, but the gun malfunctioned（故障，失灵） and two men then restrained him.

The sheriff said they had prevented a much greater tragedy.

Police searching Mr Loughner's Tucson home said they had found an envelope with messages saying "I planned ahead", "my assassination", and the name "Giffords".

So far, Mr Loughner has been charged only with offences relating to the attack on government employees, which is the responsibility of federal prosecutors.

They charged him with two counts of first-degree murder for the killings of Federal Judge John Roll and Gabe Zimmerman, an aide of Ms Giffords.

He was also charged with attempted assassination over the shooting of Ms Giffords, and two counts of attempting to murder two other aides.

The Justice Department said he would appear in court in Phoenix on Monday.

State authorities are expected to bring charges against him later for attacking non government employees.