Oscar-winning actor Philip Seymour Hoffman’s autopsy produced inconclusive results and more tests will be conducted, a spokeswoman for the New York City medical examiner’s office said Wednesday.

After the initial examination, the cause and manner of death “are pending further studies,” spokeswoman Julie Bolcer said. Toxicology reports can typically take several days to complete.

Hoffman, 46, was found dead in a Greenwich Village apartment on Sunday with a needle in his arm. Police found small bags of heroin as well as prescription medication, according to multiple media reports.

Hoffman had spoken publicly about his youthful addiction to heroin and other drugs. He had managed to stay clean for most of his adult life before suffering a relapse within the past two years. The Web site TMZ reported last year that Hoffman said he’d spent 10 days in a rehabilitation program after he had begun taking prescription painkillers and then progressed to snorting heroin.

Even addicts who have managed to remain in recovery for decades can suffer a relapse via prescription painkillers, Nora Volkow, director of the National Institute on Drug Abuse, said Wednesday.

New York police arrest four people thought to be involved with selling heroin to the late Philip Seymour Hoffman. (Reuters)

“I would like to know what was the event that triggered the relapse,” Volkow said. She said doctors need to be aware of past addictive behavior when prescribing opioid medications, which have an effect on the brain similar to heroin. “You have to be certain that you’re screening that individual for substance-use disorders,” she said.

The toxicology results could reveal whether Hoffman, like many people who overdose, was combining heroin with other drugs or medications.

Early speculation in the case centered on whether the heroin in the apartment was laced with fentanyl, a prescription narcotic that is sometimes used by heroin manufacturers to enhance the potency of the drug and that has been implicated in dozens of fatal overdoses in Maryland, Pennsylvania and other locations. A police source told the Associated Press that the heroin in Hoffman’s apartment did not contain fentanyl.

New York police arrested four people Tuesday night in lower Manhattan on what sources told the Associated Press and other news organizations were drug charges related to the Hoffman investigation.

Only one of the four, Robert Vineberg, 57, was charged with a felony, a police department spokesman said. Vineberg faces a felony charge of criminal possession of a controlled substance.