LOS ANGELES — Authorities have opened an investigation into how Matthew Perry received the supply of ketamine that killed him, police said Tuesday.

Los Angeles police are working with the U.S. Drug Enforcement Administration and the U.S. Postal Inspection Service with a probe into why the 54-year-old “Friends” star had so much of the drug in his system, LAPD Capt. Scot Williams said in an email.

An assistant found the 54-year-old Perry face down in his hot tub on Oct. 28, and paramedics who were called immediately declared him dead. His autopsy, released in December, found that the amount of ketamine in his blood was in the range used for general anesthesia during surgery. It was listed as the primary cause of death, which was ruled an accident with no foul play suspected, the report said.

Drowning and other medical issues were contributing factors, the coroner said.

The investigation was first reported by TMZ.

People close to the actor told coroner’s investigators that he was undergoing ketamine infusion therapy. The decades-old surgical drug has seen a huge surge in use in recent years as a treatment for depression, anxiety and pain.

But the medical examiner said Perry’s last treatment 1 1/2 weeks earlier wouldn’t explain the levels of ketamine in his blood. The drug is typically metabolized in a matter of hours. At least two doctors were treating Perry, a psychiatrist and an anesthesiologist who served as his primary care physician, the medical examiner’s report said. No illicit drugs or paraphernalia were found at his house.

Perry had years of struggles with addiction dating back to his time on “Friends,” when he became one of the biggest TV stars of his generation as Chandler Bing alongside Jennifer Aniston, Courteney Cox, Lisa Kudrow, Matt LeBlanc and David Schwimmer for 10 seasons from 1994 to 2004 on NBC’s megahit sitcom.

Drug-related celebrity deaths have in other cases led authorities to prosecute the people who supplied them.

After rapper Mac Miller died from an overdose of cocaine, alcohol and counterfeit oxycodone that contained fentanyl, two of the men who provided him the fentanyl were convicted of distributing the drug. One was sentenced to more than 17 years in federal prison, the other to 10 years.

Two doctors and a manager of model and reality TV star Anna Nicole Smith were charged after with conspiring to get her prescription drugs before her 2007 death, but they were not charged with causing her fatal overdose. All the counts except one misdemeanor fraud charge against one doctor were eventually dismissed.

And after Michael Jackson died in 2009 from a lethal dose of propofol, a drug intended for use only during surgery and other medical procedures, not for the insomnia the singer sought it for, his doctor, Conrad Murray, was convicted of involuntary manslaughter in 2011. Murray has maintained his innocence.