Police say the death may have been drug-related and that "prescription-type drugs" were found near his body.
Ledger was found in the bedroom of his SoHo apartment around 3:30 p.m. by his housekeeper and a masseuse who had arrived for an appointment with the actor, police say.
Ledger was nude, lying on the floor curled up, at the foot of his bed. He did not respond when they shook him, and the housekeeper called 911. He was pronounced dead at 3:45 p.m. ET.
Authorities said they did not suspect foul play but that medication was found near the body.
"There were some drugs found around the body – prescription-type drugs," a police spokesman tells PEOPLE. The spokesman says there were two bottles of prescription sleep aids, and that the prescriptions were made out to Ledger.
A medical examiner removed Ledger's body from the apartment just before 6:30 p.m. ET, and New York City Medical Examiner spokeswoman Ellen Borakove says the autopsy is scheduled for Wednesday morning.
Despite initial reports that Ledger was subletting the apartment from Mary-Kate Olsen, reps for the actress denied that Olsen ever owned or lived in the apartment.
Ledger was last spotted publicly in London on Saturday on the set of Terry Gilliam's The Imaginarium of Doctor Parnassus. Photos showed Ledger wearing a clown costume. Other pictures of Ledger out of costume show him with long brown hair and a dark goatee.
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Oscar Nomination & A Baby GirlThe Perth, Australia, native was nominated for a Best Actor Oscar for 2005's Brokeback Mountain, on whose set he met – and began a romantic relationship with – Michelle Williams. Theirs was described by others on the film as love at first sight. (PHOTOS: See a complete timeline and photos from his life.)
Ledger began his acting career in 1996, when he was 17, in an Australian TV series called Sweat, but later found big-screen stardom in Hollywood with 1999's 10 Things I Hate About You, opposite Julia Stiles. He soon followed it with films including 2000's The Patriot and 2001's A Knight's Tale and Monster's Ball.
"I'm lucky in a sense because I have a job where I get to scream and cry," he told the Chicago Sun-Times in November. "I get to purge myself in ways that don't really affect me personally. When the director yells cut, I just walk out the door and I'm back into my regular life."
In his regular life, he clearly embraced being a dad to Matilda. "Fatherhood has changed me as an artist because I feel things on a deeper level," he said. "I think my performances will grow simultaneously."
In August 2006, Warner Bros. announced that Ledger would play the Joker in The Dark Knight, a sequel to 2005's Batman Begins. On the set, he wowed his costars, including Michael Caine.
"The biggest thing in Batman will be the thing everyone was most worried about: you've got the Joker, and when Jack Nicholson was the joker, well, how are you going to top that?" Caine said last year. "Well, Heath Ledger has. He is extraordinary."
By KATHY EHRICH DOWD, STEPHEN M. SILVERMAN, RYAN PIENCIAK, ALYSSA SHELASKY and KRISTIN BOEHM
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