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Heath Ledger‘s well-documented obsession with his psychopathic role as the Joker in the Batman movie “The Dark Knight” has a new twist: He turned his Manhattan apartment into a shrine dedicated to the iconic supervillain, The Post has learned.

Police investigators who responded to the Oscar-winning actor’s Soho loft after his fatal 2008 drug overdose walked into a world filled with Batman comic books, literature on the Joker and clowns, small clown statues and recordings of Ledger practicing his oddly shifting Joker voice in high and low octaves, a law enforcement source revealed.

Ledger had immersed himself in the reading material that extended back to the very beginnings of Batman, the Joker and clown performers, the source noted.

“He was studying up on the origins of clowns and all of the previous Jokers like Jack Nicholson’s character and Cesar Romero’s, who was the first Joker on TV,” the source said.

“He was trying to make his role different from the roles of the other actors. He was even studying how to make his voice different from theirs,” the source added.

Detectives also noticed that the loft at 421 Broome St. was “immaculately clean,” with all of Ledger’s research on the Joker neatly stacked, the source said.

“He was a perfectionist who clearly had a lot of respect for the character. All of that stuff was very tidy. He cared a lot about it,” the source recalled.

Ledger’s official cause of death was “acute intoxication by the combined effects of oxycodone, hydrocodone, diazepam, temazepam, alprazolam, and doxylamine,” according to a report by the New York City medical examiner’s office.

“We have concluded that the manner of death is accident, resulting from the abuse of prescription medications,” including painkillers, sleeping pills and anti-anxiety drugs, the report said.

Ledger’s nude body was discovered on his bed by a masseuse who had arrived at his loft for a scheduled appointment. He was 28 at the time of his death on Jan. 22, 2008.

Later that year, a documentary on Ledger titled “Too Young to Die” was released, focusing on a chilling diary that the actor kept during the filming of “The Dark Knight,” which starred Christian Bale as Batman. (The movie, also released in 2008, became an instant box office smash, grossing over $1 billion.)

Ledger’s red-and-blue, bound journal with “The Joker” emblazoned on the front was composed of ominous photos, comic strips and drawings of the movie villain, clowns and hyenas. There was also a picture of Malcolm McDowell playing the brutish character Alex DeLarge in “A Clockwork Orange.”

On the back of one of the pages, Ledger made a disturbing notation, scrawling the words, “bye bye.”

Before his death, Ledger was interviewed by Empire Magazine and described how his interpretation of the Joker was born.

“I sat around in a hotel room in London for about a month, locked myself away, formed a little diary and experimented with voices — it was important to try to find a somewhat iconic voice and laugh,” he said.

“I ended up landing more in the realm of a psychopath — someone with very little to no conscience towards his acts,” Ledger said. “He’s just an absolute sociopath, a cold-blooded, mass-murdering clown … Nothing intimidates him, and everything is a big joke.”