NEW YORK -- The sudden death of Hollywood heartthrob and rising Australian star Heath Ledger remained a mystery Wednesday as experts began a second round of toxicology tests after an initial autopsy was inconclusive.

The 28-year-old, who shot to worldwide renown with his sensitive portrayal of a repressed homosexual cowboy in the Oscar-winning "Brokeback Mountain," was found naked and unresponsive in his Manhattan apartment on Tuesday.

Police said that prescription medications were found in the residence and that a rolled-up 20 dollar bill found near the body had tested negative for drug residue. The Australian actor's family insisted his death was "accidental."

"The autopsy is inconclusive," New York City medical examiner's office spokeswoman Ellen Borakove said. "We have to do additional testing which will include toxicology and tissue testing. It will take about 10 days."

Prescription pills bearing the name of the Australian actor were found in the apartment, police said. The entertainment news website TMZ said they included anti-anxiety drugs Xanax and Valium as well as the sleep aid Ambien.

New York City Police Commissioner Ray Kelly told a news conference that no illegal drugs were found in the apartment. Police tested a rolled bill and found no evidence of illicit substances on it, US media reported.

Ledger had separated in September from his fiancee Michelle Williams, who played the wife of his cowboy character Ennis Del Mar in 2005's "Brokeback Mountain," and who is the real-life mother of his daughter, Matilda Rose, 2.

The film's Oscar-winning director Ang Lee described the actor's sudden death as "heartbreaking" and said working with him had been "one of the purest joys of my life," according to a statement released to People.com.

"He brought to the role of Ennis [Del Mar] more than any of us could have imagined -- a thirst for life, for love, and for truth, and a vulnerability that made everyone who knew him love him. His death is heartbreaking."

Ledger's publicist Mara Buxbaum appealed for privacy for Ledger's family.

"This is an extremely difficult time for his loved ones. We're asking the media to please respect the family's privacy and avoid speculation until the facts are known," said Buxbaum.

The actor's father Kim Ledger read a statement outside the family home in Perth, Australia, indicating the family's belief was that Ledger's death was "very tragic, untimely and accidental," and not a suicide.

Ledger was found dead Tuesday in an apartment in the posh district of SoHo, a police spokesman said. Neighbors said Ledger had been renting the loft for the past several months.

New York City deputy police commissioner Paul Browne said a masseuse and a housekeeper discovered his body after the masseuse arrived for an appointment.

At 19, Ledger left Sydney for Hollywood, where his talent was spotted by Mel Gibson when auditioning 500 actors for the role of his son in "The Patriot" -- a break that led to his leading role in "A Knight's Tale."

Gibson, who plucked Ledger from relative obscurity, said the death of the versatile actor was a "tragic loss."

"I had such great hope for him," the Oscar-winning actor and director said.

Australia's Prime Minister Kevin Rudd said it was "tragic that we have lost one of our nation's finest actors in the prime of his life ... Heath Ledger's diverse and challenging roles will be remembered as some of the great performances by an Australian actor."

Ledger was nominated for a best actor Oscar for his "Brokeback Mountain" role. He had been working this month on "The Imaginarium of Doctor Parnassus," directed by Terry Gilliam, which was due out next year. He also portrays The Joker in the upcoming Batman movie "Dark Knight."