A federal grand jury has indicted suspected Arizona shooter Jared Loughner on three counts for trying to kill US lawmaker Gabrielle Giffords and two of her aides, officials said Wednesday.

"We are in the early stages of this ongoing investigation. We have made considerable progress in a short period of time," US Attorney Dennis Burke said in a statement after the indictment was returned in Tucson.

"This case also involves potential death-penalty charges, and (Justice) Department rules require us to pursue a deliberate and thorough process. Today's charges are just the beginning of our legal action, and we are working diligently to ensure that our investigation is thorough and that justice is done for the victims and their families."

Loughner, 22, was arrested after the shooting at a public meeting Giffords held in Tucson on January 8, which triggered a wave of soul-searching over America's starkly divisive political culture.