It could take weeks for investigators to fully determine Loughner's state of mind in the days leading up to the rampage, the sources said. Mr Loughner appeared in court in Phoenix at 1400 local time (2100 GMT). If convicted, he could face the death penalty.
Court documents show he has been charged with:

• Attempting to assassinate Ms Giffords
• Killing two government officials, federal judge John Roll and Gabe Zimmerman, an aide of Ms Giffords
• Attempting to kill two more government officials

State authorities are expected to bring charges against him later for attacking non-government employees.
Mr Loughner has not co-operated with investigators, instead invoking his right under the US constitution to remain silent.
Local Sheriff Clarence Dupnik said he almost certainly acted alone.

"If the tragedy in Tucson was not enough to ensure that Congress finally takes action, we hope this clear call for reform from the public will add to the groundswell of support."