Washington: A note written by Jared L. Loughner ahead of his shooting rampage on Saturday in Tucson, Arizona appears to show that he had planned to assassinate US congresswoman Gabrielle Giffords, a media report said.

Prosecutors charged Loughner, a troubled 22-year-old college dropout, with five federal counts Sunday, including the attempted assassination of Giffords in connection with the shooting that left six people dead and 14 wounded, The New York Times reported.

Evidence seized from Loughner's home, about 8 km from the shooting scene, indicated that he had planned to kill Giffords, Democrat of Arizona, according to documents filed in Federal District Court in Phoenix, the capital of Arizona.

Special Agent Tony M. Taylor Jr. of the FBI said in an affidavit that an envelope found in a safe in the home bore these handwritten words: "I planned ahead", "My assassination" and "Giffords".

Loughner, who is believed to have acted alone, is in federal custody and is scheduled to make his first court appearance before a magistrate judge in Phoenix on Monday.

Investigators focused their attention on Loughner, whom they accused of methodically planning the shootings, which occurred outside a supermarket. The court documents said Loughner bought the semiautomatic pistol Nov 30 in Tucson.

The gun was legally purchased, officials said, prompting criticism of the state's gun laws, which allow the carrying of concealed weapons.

The documents also indicated that the suspect had previous contact with the congresswoman. A letter from Giffords was also found in the safe at Loughner's home, thanking him for attending a 2007 "Congress on Your Corner" event, like the one she was holding Saturday morning when she was shot.

According to the report, Giffords was in critical condition after surviving, against the odds, a single gunshot wound to the head at point-blank range. Her doctors were cautiously optimistic that she would survive, and said Sunday that they had removed nearly half of her skull to prevent damage from the swelling of her brain.

Robert S. Mueller III, the director of the FBI, travelled to Tucson to oversee the shooting investigation. He said an intensive investigation was seeking to determine "why someone would commit such a heinous act and whether anyone else was involved".

Mueller added that discussions were under way to increase security for all members of Congress.

Capitol security agencies are planning to join the FBI Wednesday in a security briefing for members of Congress. Already, the US Marshals Service has increased protection for federal judges in Arizona.

Along with being accused of trying to kill Giffords, Loughner was charged with the killing and attempted killing of four government employees: John M. Roll, the chief federal judge in Arizona, who was killed; Gabriel Zimmerman, a Congressional aide, who was also killed; and Pamela Simon and Ron Barber, aides who were wounded. Loughner could face the death penalty if convicted.

Mueller said additional state charges might be filed, and he did not rule out the filing of terrorism charges.

According to the daily, Loughner has refused to cooperate with investigators and has invoked his Fifth Amendment rights, the Pima County sheriff's office said.

Judy Clarke, a federal public defender who has handled major cases, has been appointed to represent Loughner, CNN reported.

An outpouring of grief was on display all over Tucson, where friends of the victims joined complete strangers in lighting candles and offering tear-filled prayers. President Barack Obama called on Americans to observe a moment of silence at 11 a.m. Monday in honour of the wounded and dead.