Perry, who was best known for playing wise-cracking Chandler Bing in Friends, died at his LA home last October after being found unresponsive in a swimming pool.

A post-mortem found his death was an accident from "the acute effects of ketamine".

Ketamine is a sedative that can be used as a recreational drug, as well as to treat depression.

Read more: Matthew Perry - A life in pictures

Los Angeles Police Department says it is working with the Drug Enforcement Agency as part of an investigation into why Perry, 54, had so much ketamine in his system at the time of his death.

People close to Perry told investigators that he was undergoing ketamine infusion therapy - an experimental treatment - according to his autopsy.

The medical examiner wrote however that Perry's last treatment was one and a half weeks before his death and would not explain the levels of ketamine in his blood.

Perry, who was 54, had also drowned in "the heated end of his pool" in what the medical examiner described as a secondary factor in his death.

They added he had "reportedly been clean for 19 months". Perry regularly spoke about his battle with addiction - including a near-death experience in 2019 after his colon burst as a result of opioid use.

Perry recalled one instance when fellow Friends star Jennifer Aniston confronted him about being inebriated while filming.

"I know you're drinking," he remembered her telling him.

"We can smell it," she said in what Perry called a "kind of weird but loving way" - adding: "The plural 'we' hit me like a sledgehammer."

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Speaking to NBC's Today presenter Hoda Kotb on her Making Spaces podcast in March, Perry's stepfather said the star "felt like he was beating" his battles with addiction.

Keith Morrison, an award-winning correspondent for Dateline NBC, said his stepson "didn't get to have his third act, and that's not fair".