Federal investigators found evidence suggesting that 22-year-old Jared Lee Loughner went to a Tucson Safeway on Saturday with the intent of assassinating Rep. Gabrielle Giffords (D-Ariz.).

In an affidavit released Sunday, federal investigators said they discovered a handwritten note inside a safe in Loughner’s Tucson home. According to the document, the words “I planned ahead,” “my assassination” and “Giffords” were written on an envelope, along with Loughner’s signature.

Giffords was shot once in the head when a gunman opened fire on a constituent event she was holding outside the grocery store. She is in critical condition at a Tucson hospital. Six people were killed in the attack: U.S. District Judge John Roll; Gabe Zimmerman, a 30-year-old Giffords aide who was recently engaged; Christina Taylor Green, a 9-year-old girl who had just been elected to her school’s student council; Dorothy Morris, 76; Dorwin Stoddard, 76; and Phyllis Scheck, 79.

Loughner was arrested at the scene. He is expected to appear before a judge in Arizona on Monday afternoon.