An autopsy on Heath Ledger was inclusive, and more tests are needed, the medical examiner's office said Wednesday, a day after the 28-year-old actor was found dead with sleeping pills nearby.

It will take about 10 days to complete the investigation, said Ellen Borakove, a spokeswoman for the medical examiner.

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Earlier, police said the death was caused by a possible drug overdose and appeared to be accidental.

Ledger was known for grueling, intense roles that became his trademark after he got his start in teen movies like 10 Things I Hate About You.

In what may be his final finished performance, he took a rare role in a guaranteed summer blockbuster, playing Batman's nemesis, the Joker, in the upcoming The Dark Knight. But the role was nothing he could phone in; it forced him to re-brand a character last played on the big screen by Jack Nicholson.

The actor's personal strife was accompanied by professional anxiety.

Ledger said in an interview in November that The Dark Knight and last year's I'm Not There, took a heavy toll. He said he "stressed out a little too much" during the Dylan film, and had trouble sleeping while portraying the Joker, whom he called a "psychopathic, mass-murdering, schizophrenic clown with zero empathy."

"Last week I probably slept an average of two hours a night," Ledger told The New York Times. "I couldn't stop thinking. My body was exhausted, and my mind was still going." He said he took two Ambien pills, which only worked for an hour.

The film's director, Christopher Nolan, said that Ledger's Joker would be wildly different from Nicholson's. "It was a very great challenge for Heath," Nolan said. "He's extremely original, extremely frightening, tremendously edgy. A very young character, a very anarchic presence that taps into a lot of our basic fears and panic."

Ledger was currently filming for Terry Gilliam's The Imaginarium of Doctor Parnassus.