I don't think they're sure yet, but there was an autopsy done. Nothing immediately obvious, and I guess they said there was no sign of drugs or alcohol, but who knows.. once they do the toxicology tests and release the autopsy report we will know.
I feel so sorry for his family..
One article I read, I think it may have been this TV guide one that was linked in the first post, in the comment section this girl said she was best friends with Nolan's sister (she could have been BS'ing but why would someone do that in this circumstance?) and that Nolan was a wreck
they had grown really close throughout the show, which was pretty obvious and yeah... I can only imagine how difficult this is for those who were close to him, he had so many things going for him and was so young.
RIP Todd