(AP) -- Four people were taken into custody on drug charges after police investigating Philip Seymour Hoffman's death executed search warrants, two people with knowledge of the investigation said Wednesday, and the medical examiner's office said more tests are needed to determine what killed him.

There was no timetable for Hoffman's autopsy to be finished, said medical examiner office spokeswoman Julie Bolcer, who declined to discuss the pending tests. Toxicology and tissue tests are typically done in such cases.

Police have been investigating his death as a suspected drug overdose.

The Oscar-winning actor was found dead Sunday with a needle in his arm, and tests found heroin -- but no traces of the potent synthetic morphine additive fentanyl, which is added to intensify the high and has been linked to 22 suspected overdose deaths in western Pennsylvania -- in samples from at least 50 packets in his apartment in Manhattan's Greenwich Village, law enforcement officials have said.

The four people were taken into custody Tuesday night after police executed search warrants at several New York City apartments based on a tip provided by a confidential source that the suspects may have supplied Hoffman with drugs, according to two people with knowledge of the investigation who spoke to The Associated Press on the condition of anonymity because investigators have not obtained evidence to corroborate the reported connection.

Police say undisclosed quantities of heroin and marijuana were found Tuesday night in three apartments in a lower Manhattan building. The four suspects -- three of whom live in the building -- face charges of criminal possession of a controlled substance. Two also face charges of criminal use of drug paraphernalia. They're awaiting arraignment in Manhattan Criminal Court.

Investigators have determined that the "Capote" star made six ATM transactions for a total of $1,200 inside a supermarket near his home the day before his death, law enforcement officials have said. Investigators have been piecing together Hoffman's final hours using video surveillance to determine his whereabouts. They're also examining a computer and two iPads found at the scene for clues.

The NYPD's intensive effort to determine the source of the drugs in an apparent accidental overdose is unusual. Courts have found that under state law, drug dealers cannot be held liable for a customer's death.

Hoffman had been frank about struggling with substance abuse. He told CBS' "60 Minutes" in 2006 that had he used "anything I could get my hands on" before getting clean at age 22. But in interviews last year, he said he had relapsed, had developed a heroin problem and had gone to rehab for a time.

Hoffman's relatives said they were devastated by a death both "tragic and sudden." His funeral is scheduled for Friday.

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Associated Press writer Jennifer Peltz contributed to this report.