(CNN) -- Federal drug investigators have taken over the inquiry into the January 22 death of actor Heath Ledger in a New York apartment, a law enforcement source told CNN Thursday.



Actor Heath Ledger, 28, died January 22 at an apartment in Lower Manhattan.



Ledger, 28, an Oscar-nominated Australian actor, died of an accidental overdose of six types of medication, according to the New York City medical examiner's office.



The medical examiner's office said Ledger "died as the result of acute intoxication by the combined effects of oxycodone, hydrocodone, diazepam, temazepam, alprazolam and doxylamine."



Oxycodone and hydrocodone are pain medications; diazepam -- also known as Valium -- is used to treat anxiety; temazepam is a sedative used to induce sleep; alprazolam -- known by the trade name Xanax -- is an anti-anxiety agent. Doxylamine, an antihistamine, can be obtained over the counter as a sleep aid.



The law enforcement source said the federal drug investigators are trying to determine if the drugs were prescribed legally, and are looking at two doctors -- one based in California and another in Texas.



Ledger's first American film was the teen comedy "10 Things I Hate About You" in 1999. He passed up several scripts before taking a role in the Revolutionary War drama "The Patriot" in 2000 and "A Knight's Tale" in 2001. He also played a supporting role in "Monster's Ball."



But Ledger was perhaps best known for his portrayal of Ennis Del Mar in "Brokeback Mountain," Ang Lee's film about two cowboys who had a secret romantic relationship. The role earned Ledger a best actor Oscar nomination. E-mail to a friend



CNN's Brian Vitagliano contributed to this report.
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