The January 8, 2011, attack killed six people and wounded 13 others, including Giffords, who was holding a meet-and-greet event with constituents.

CNN: STORY HIGHLIGHTS

The shooting occurred during a meet-and-greet event for Giffords.

CNN: Attorneys for Loughner expected to enter new plea Tuesday

Court documents released Sunday show that investigators found a letter from Giffords in a safe at the house where Loughner lived with his parents, thanking him for attending a 2007 event, similar to Saturday's meet-and-greet.

CNN: STORY HIGHLIGHTS