Gabrielle Giffords was transferred to a rehabilitation facility in Houston, Texas, following life-saving surgery [EPA]

The Arizona shooting suspect Jared Lee Loughner has pleaded not guilty to charges he tried to kill Democratic Representative Gabrielle Giffords and two of her aides.

The plea on monday by Loughner marked his second court appearance since he allegedly shot the congresswoman and 18 others at Gifford's meet-and-greet event on January 8 outside a grocery store in Tucson. Six people died, including US District Judge John Roll and a 9-year-old girl. Thirteen others were wounded.

Loughner, 22, faces federal charges of trying to assassinate Giffords and attempting to murder two of her aides. He will later face state charges dealing with other victims.

Mentally disturbed

Investigators have said Loughner was mentally disturbed and acting increasingly erratic in the weeks leading up to the shooting.

If his attorney uses mental competency questions as a defence and is successful, Loughner could be sent to a mental health facility instead of being sentenced to prison or death.

US District Judge Larry Burns of San Diego asked Loughner attorney Judy Clarke whether there was any question about her client's ability to understand the case against him.

"We are not raising any issues at this time," Clarke said.

Prosecutor Wallace Kleindienst estimated that he would know within the next 30 days whether additional federal charges would be filed against Loughner.

Kleindienst said prosecutors provided defence lawyers with records taken from Loughner's computer and documents of about 250 interviews made in the case.

The judge did not rule on prosecutors' request to move the federal case back to Tucson so that victims and witnesses do not have to make the four-hour round trip drive to Phoenix to attend court hearings.

Clarke said she did not oppose the request, but questioned where Loughner would be jailed in Tucson if the case were moved.

Last Friday, Giffords was transferred to a rehabilitation facility in Houston, Texas, following life-saving surgery and intensive care at the University Medical Center in Tucson in the days after the shooting.