Results of a post-mortem examination carried out on Hollywood star Heath Ledger are inconclusive, US authorities have said.

The 28-year-old Brokeback Mountain star was found dead at his New York apartment on Tuesday afternoon.

Police are still treating his death as possibly drug-related and said sleeping pills were found close to his body.

But a spokeswoman for the medical examiner’s office said an autopsy had proved inconclusive and further tests will be required – which could take around 10 days.

There have been unconfirmed reports that the actor was suffering from pneumonia at the time of his death.

His family say his death was accidental.

Heath spent his last days shooting a film in London. He was working on the new Terry Gilliam project The Imaginarium of Doctor Parnassus and gave no outward signs of being troubled, sources said.

An insider on the film said: “This news has come as a terrible shock. There were no problems we were aware of.

“He didn’t chat much on set but he was always polite to the cast and crew. He wasn’t keen on all the publicity that goes with a film and just wanted to concentrate on his acting.”