Federal drug authorities are investigating two doctors regarding Heath Ledger's death, the New York Daily News reports.

Doctors in Texas and California are being probed concerning the prescriptions - namely Oxycontin and Vicodin - strong painkillers found in the actor's system at the time of his death last month.

A law enforcement source told the Daily News on Tuesday that the investigation is ongoing. "It's not clear if there was any wrongdoing," the source told the paper.

Ledger, 28, was found dead in his Manhattan apartment on January 22, and after an autopsy and toxicology report, the medical examiner ruled his death an accidental prescription drug overdose. According to the paper, the federal Drug Enforcement Administration has issued a subpoena of those results as part of its investigation.

Law enforcement officials are working to determine whether the painkillers were prescribed legally.

Heath Ledger will be laid to rest in a $25,000 mahogany casket lined with cream-colored velvet in his native Perth, Australia on Friday, February 1, according to reports.

The body of the actor, who was found dead in his New York apartment last week, was released by a funeral home in the city after an autopsy last week.

Family and friends paid tribute to the tragic star at an intimate memorial service in Los Angeles last weekend.

Heath Ledger's father paid an emotional visit to the late actor's New York apartment on Tuesday, a week after the star was found dead there.

Kim Ledger arrived at the luxury Broome Street, Soho pad accompanied by police detectives, and spent an hour inside.

According the PageSix.com, Ledger Sr. left through a back entrance, so he could see a private memorial for the 28-year-old, which had been erected in the building's garden by other residents.

On Saturday, Kim stood alongside family and close friends at a private service for the Brokeback Mountain star at the Westwood Village Memorial Park in Los Angeles.

The Oscar-nominee will be buried in his hometown of Perth, Australia.