Bottles of the generic forms of Xanax and Valium, both anti-anxiety drugs prescribed in his name, were found at his $26,000-a-month, three-bedroom loft apartment, the New York Post reported. (Map: Ledger's Soho address)
Police also found sleeping pills Donormyl, Zopiclone, and Ambien, the Post reported, quoting a police source who said: "it does not appear to be a suicide".
The 28-year-old actor was found nude, lying face down and unresponsive by his maid, Theresa Solomon, and masseuse Diane Lee at 3.20pm local time (7.20am AEDT) .
Ms Solomon said she had heard the actor snoring at midday.
The maid and therapist called 911 at 3.26pm. Emergency workers were unable to revive the star and he was pronounced dead at 3.35pm.
"We are investigating the possibility of an overdose," NYPD spokesman Paul Browne said.
"There were pills within the vicinity of the bed."
One of the drugs at the scene, Ambien, is marketed as Stilnox in Australia and hit headlines last year when a national drug hotline fielded calls from users reporting strange side-effects. (Related article: Ledger's sleeping pill 'controversial')
In an interview with the New York Times last November, Ledger said he'd been taking Ambien which, according to the newspaper, "failed to work".
"He took a second one and fell into a stupor, only to wake up an hour later, his mind still racing," the journalist reported. Some reports said Ledger was sick with pneumonia before he died.
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