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
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."