You can read the five-count criminal complaint against Jared Loughner here. The complaint references a “digital surveillance video depicting events at the Safeway” and concludes that the incident was a pre-meditated assassination attempt, based on materials seized from Loughner’s residence:

Some of the evidence seized from that located included a letter in a safe, addressed to “Mr. Jared Loughney” at 7741 N. Soledad Avenue, from Congresswoman Giffords, on Congressional stationary, dated August 30, 2007, thanking him for attending a “Congress on your Corner” event at the Foothills Mall in Tuscon. Also recovered in the safe was an envelope with handwriting on the envelope stating “I planned ahead,” and “My assassination,” and the name “Giffords,” along with what appears to be Loughner’s signature.

Via Dave Weigel.