I've learned a few things already, so I'll give you what I have. The most likely culprit is someone called Kadaj. He's got silver hair like Aang described... and he's crazy. He's obsessed with finding something he calls his 'mother', and he'll do anything to find her -- anything, even killing as many people as he can until he does. He has a brother called Loz; it's probably safe to assume they're working together.

The main problem is finding him. I don't know where he is now, or even if he was the one who attacked Aang for sure. But if he isn't stopped, a lot of people could get hurt.