Federal drug authorities are investigating two doctors, one in California and one in Texas, in regards to Heath Ledger's tragic death last month, People reported.

According to an autopsy, the 28-year-old Australian actor, who was found unresponsive in his New York apartment late last month, died of an accidental overdose of six prescription drugs, including the two painkillers, sleeping pills and anti-anxiety medication.

Authorities are now interested in finding out if the drugs were illegally prescribed. The two doctors are believed to have supplied Ledger with two of the drugs involved in his death, Oxycontin and Vicodin.

"We are investigating doctors in Los Angeles and Texas with regard to Ledger’s prescriptions," a source said.

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."

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.

Immediately after toxicology reports revealed the cause of Heath's death on Feb. 6, the Drug Enforcement Administration announced it was launching an investigation into how the actor obtained the various drugs.

Three days later, several close friends and family members, including Ledger's parents, sister, former fiancée Michelle Williams and former costar Cate Blanchett gathered in Ledger's hometown, Perth, Australia for a private memorial.