The Brokeback Mountain star was found dead at his apartment in Manhattan, New York last Tuesday.
The New York Police Department confirmed that drugs had been found in the apartment, but a 90-minute autopsy conducted last week was inconclusive.
Toxicology and tissue tests are currently being carried out to determine the cause of Ledger's death.
It has been suggested that he may have died of natural causes because the level of toxicity in his system caused by medication is arguably too low to have killed him, according to TMZ.
Authorities investigating Ledger's death have also ruled out reports claiming that the actor's masseuse was involved in a plot with Mary-Kate Olsen's bodyguard to remove illegal drugs from the room.
Officers insisted that they were with the bodyguard the entire time and there would have been no time to carry out such a plan.
A private memorial service was carried out for Ledger in Los Angeles on Saturday. His former girlfriend Naomi Watts and ex-fiancée Michelle Williams and their two-year-old daughter were present.