Tests have confirmed there was heroin in at least some of the scores of plastic packets in the New York apartment where actor Philip Seymour Hoffman was found dead.
Authorities are now working to determine whether the drug was mixed or tainted with anything else.
Medical examiners have not yet made an official determination of the cause of the 46-year-old actor's death, but police have been investigating it as a suspected overdose. Hoffman was found in a bathroom with a syringe in his arm, officers said.
Video: Fans react to the actor's death
Details have begun to emerge of Hoffman's final day and the circumstances in which he was found at home in Manhattan's Greenwich Village on Sunday.
Speculation has persisted over whether Hoffman's death could be linked to a potent blend of heroin and synthetic morphine that has been connected to deaths elsewhere, although there are so far no official findings pointing to that scenario.
"The direction of the investigation is going to depend, in large part, on the findings of the medical examiner and the findings of the lab tests," chief police spokesman Stephen Davis said.
An autopsy began on Monday, but results were not expected until at least yesterday.
There were at least four dozen small packets in the apartment, law enforcement officials said.