Did Heath Ledger have a substance abuse problem?

After cops found actor Heath Ledger dead in his Soho apartment on Tuesday, sources have confirmed that Ledger had a substance abuse problem. TMZ is reporting that he had been clean for a year.
Ledger's body was evidently found dead in his Soho apartment face down on a bed surrounded by sleeping pills when his housekeeper found him.
Police say it is too early to determine whether the death was accidental or suicide.
Heath Ledger was 28 years old at the time of his death.