Nick Baumann at Mother Jones spoke to a Jared Loughner friend (via ben smith), who says Loughner's anger at Rep. Gabrielle Giffords was triggered after she didn't answer a question satisfactorily at a similar 2007 event (the question was reportedly What is government if words have no meaning?'):

Giffords' answer, whatever it was, didn't satisfy Loughner. "He said, 'Can you believe it, they wouldn't answer my question,' and I told him, 'Dude, no one's going to answer that,'" Tierney recalls. "Ever since that, he thought she was fake, he had something against her."

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Loughner would occasionally mention Giffords, according to Tierney: "It wasn't a day-in, day-out thing, but maybe once in a while, if Giffords did something that was ridiculous or passed some stupid law or did something stupid, he related that to people. But the thing I remember most is just that question. I don't remember him stalking her or anything." Tierney notes that Loughner did not display any specific political or ideological bent: "It wasn't like he was in a certain party or went to rallies…It's not like he'd go on political rants." But Loughner did, according to Tierney, believe that government is "f***ing us over." He never heard Loughner vent about about the perils of "currency," as Loughner did on one YouTube video he created.

CNN adds:

Investigators found a letter from the congresswoman in a safe at the house where Loughner lived with his parents, thanking him for attending (the) 2007 event, according to court documents released Sunday.

"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," the affidavit states.

Loughner's behavior at community college in Tucson was recorded in emails by one of his fellow students. You can read them here.

"We have a mentally unstable person in the class that scares the living crap out of me. He is one of those whose picture you see on the news, after he has come into class with an automatic weapon. Everyone interviewed would say, Yeah, he was in my math class and he was really weird. I sit by the door with my purse handy. If you see it on the news one night, know that I got out fast…"