As noted below (and more at this link), the man who bought the Nissan Pathfinder has been identified, and he's a naturalized Pakistani.
Bloomberg has a hunch, though:
"If I had to guess 25 cents, this would be exactly that. Homegrown, or maybe a mentally deranged person, or somebody with a political agenda that doesn't like the health care bill or something. It could be anything," he said.