Two doctors are being investigated in connection with the prescriptions Heath Ledger was taking when he died in January this year.
The US Drug Enforcement Agency is probing who, and why, prescribed the combination of drugs that it is believed to have killed the Aussie actor in his sleep.
According to reports in the New York Daily news, Federal drug investigators have focused their attention on medics in Texas and California who are believed to have supplied the actor with OxyContin and Vicodin.
The paper says that it is not known whether the doctors prescribed both drugs or just one each to the late actor.
"It's an ongoing investigation. It's not clear if there was any wrongdoing," Usmagazine.com quoted a law enforcement source, as telling the paper.
A rep for the Drug Enforcement Administration had revealed earlier this month that they were launching an investigation into how Ledger got the prescription drugs.
"We're trying to educate that legal drugs - these prescription drugs - can be just as dangerous as more hardcore drugs when misused or abused," Steve Robertson, a special agent with the DEA, said at the time.