I'm noticing that on the news, the constant refrain is that law enforcement is "trying to discover why" he did it. What? He already said why he did it. Take him at his word.
Look at the news story:
http://news.google.com/news?hl=en&ned=&ie=UTF-8&ncl=http://abcnews.go.com/US/wireStory%3Fid%3D1687598
Some of them don't even MENTION his nationality.