TUCSON, Arizona (Reuters) - A federal grand jury in Arizona indicted Jared Lee Loughner on Wednesday for attempting to assassinate Democratic Representative Gabrielle Giffords and attempted murder of two of her staff members.

Loughner, 22, is accused of going on a shooting spree outside a grocery store in Tucson on January 8 that killed six people and injured 13 others. He is expected to face additional federal and state charges.

"Today's charges are just the beginning of our legal action. We are working diligently to ensure that our investigation is thorough and that justice is done for the victims and their families," U.S. Attorney Dennis K. Burke said in a statement released in Washington.

The initial charges against Loughner carry a maximum penalty of life in prison. If indicted later for murder, he could face the death penalty.