I heard a story on the Michael Savage show a couple of weeks ago about a teenage girl who died like this too. The parents were devastated.
I have a friend in the FBI who says they have a huge file of investigations along these lines....unfortunately, kids and adults do it all the time. Parents get these kooky ideas that it was some kind of ritual killing or that someone else was involved. Then they bring in an expert who has the awful job of showing the parents (if they persist) the hundreds of other files on kids and adults who do this to themselves......UGH.
My local library had a web site for "kids and teens" that pointed to an American Library Association web site that pointed to a web page designed for kids and teens to learn about things including autoerotic asphyxiation (cutting off air to get a better orgasm). I complained and they said they would take action but they never did. Ultimately the ALA stopped pushing the web site.