[PHOTOS: Remembering Philip Seymour Hoffman]

(TMZ) - Oscar-winning actor Phillip Seymour Hoffman was found dead Sunday in his New York City apartment of a reported drug overdose. He was 46.

TMZ reports that authorities found a needle in his arm. Law enforcement sources tell TMZ the actor was found Sunday morning inside of his Greenwich apartment. An assistant found the actor alone in the apartment -- he hadn't heard from Hoffman and was checking up on him.

Many details surrounding his death are unclear, but the actor had struggled with drug problems, most recently checking himself into a rehab facility for heroin back in May. The New York Post is reporting heroin was found in the apt and Hoffman was on the bathroom floor in his underwear when he was found.

TMZ broke the story of Hoffman going to rehab as a preventative measure because he was getting close to a major relapse.  A source involved in Hoffman's treatment now tells TMZ... the actor said at the time he was on the brink of becoming a heroin IV user and wanted to nip it in the bud before that happened. As previously reported, Hoffman had been clean for 23 years but fell off the wagon in 2012. 

PSH was spotted at the Sundance Film Festival on January 19th ... but didn't have much to say when he was filmed by photogs.

Hoffman won the Oscar for Best Actor in 2006 for his role as Truman Capote in the biographical film Capote. 

Most recently, he played Plutarch Heavensbee in The Hunger Games: Catching Fire. According to IMDB, filming of The Hunger Games: Mockingjay -- Part 1 is complete, but the franchise finale, The Hunger Games: Mockingjay -- Part 2 is still in production. Sources familiar with Hoffman's role in the film say he had completed filming for all but one scene in part one of the next film.  The actor had about a week of filming left in part two, and the producers expect to rewrite a scene to replace his character.  The next Hunger Games movie is set for release this November. 

According to PeopleHoffman leaves behind longtime partner, Mimi O'Donnell, and their three children.

Hoffman's family released a statement saying, "We are devastated by the loss of our beloved Phil and appreciate the outpouring of love and support."

Law enforcement sources tell say an autopsy will take place Monday.