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Philip Seymour Hoffman was found dead Sunday in a New York City apartment of an apparent drug overdose ... TMZ has learned authorities found a needle in his arm.

Law enforcement sources tell TMZ ... the actor was found Sunday morning inside a Greenwich apartment ... and they're still on scene investigating.

Our sources say ... Hoffman was found in his bathroom in his boxer shorts, with a needle sticking out of his left arm. We're told police found 10 glassine envelopes -- 2 of the envelopes had heroin ... the others presumably were empty.

One of the envelopes had "Ace of Spades" written on it, and another had an ace of hearts symbol -- such symbols and words are often markers from people who make heroin -- to brand it.

Law enforcement sources tell TMZ ... Hoffman was supposed to pick up his kids at 9 AM Sunday but didn't show -- which was unusual for him. A playwright and a friend became concerned, came over and found the actor alone in the apartment.

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.

TMZ broke the story of Hoffman going to rehab. We were told he did it 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 we previously reported ... Hoffman had been clean for 23 years but fell off the wagon in 2012.

It appears Hoffman's last interview was at the Sundance Film Festival with XiXi Yang from PopStopTV for his movie "A Wanted Man."

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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."