The Australian actor, 28, was found dead in his downtown New York apartment on Jan. 22. An autopsy whose results were announced earlier this month blamed an accidental mixture of prescription drugs, said New York City's Chief Medical Examiner.
“We are investigating doctors in Los Angeles and Texas with regard to Ledger’s prescriptions,” the source tells PEOPLE.
However, the source did not say whether any wrongdoing is involved, saying, “We don’t know yet. The agents were there this week, and we are waiting to hear from them.”
The doctors in question are thought to have supplied Ledger with Oxycontin and Vicodin, law enforcement sources tell New York's Daily News.
At issue is whether or not the powerful painkiller were prescribed legally, says the newspaper.
DEA SubpoenasThe subpoenaing of Ledger's autopsy results by the Drug Enforcement Administration was reported shortly after the findings of the report were announced. On Feb. 6, it was said that the DEA would look into the source of the actor's prescriptions.
Besides Oxycodone and Vicodin (also known as Hydrocodone), autopsy results also showed that Ledger's system contained Diazepam (commonly called Valium), as well as Temazepam, which treats anxiety or sleeplessness; Alprazolam (also known as Xanax); and Doxylamine, a sedating antihistamine often used as a sleeping aid.
On Feb. 9, Ledger's former girlfriend, Michelle Williams – who is mother of his 2-year-old daughter, Matilda – joined Cate Blanchett, Ledger's family and hundreds of mourners at a memorial in Australia for the actor. On Jan. 26, a private memorial in Los Angeles was held for close friends and family.