If he is an American citizen, the media will tell us that probably some time today. If they don't say anything about it, it means he's probably an illegal.
Here is a his picture, looks like it a picture from a high school yearbook.
https://pbs.twimg.com/media/Be7Q0AdIgAEHuqo.jpg
http://www.heavy.com/news/2014/01/darion-marcus-aguilar-columbia-mall-shooter/