Matthew Perry’s death has, publicly, been considered an open and shut case — but, as far as the authorities go, they’re still digging into how he got one particular drug … TMZ has learned.

Law enforcement sources tell us there is an ongoing investigation into where Matthew got the ketamine that ended up playing a part in his death — and the main questions are who provided the drug, and under what circumstances.

Our sources say both local police and the DEA have been looking into this matter for months, but it’s unclear where exactly investigators are in the probe.

We do know they’ve already interviewed some key people in Hollywood.

While our sources could not provide names … they told us investigators have approached and spoken to people who are known to have a history of drug use or abuse.

We’re told those people are not necessarily the actual source of ketamine, but they could have information leading to the source. So far, no arrests have been made.

TMZ broke the story … Matthew’s assistant found him unresponsive in the hot tub at his Pacific Palisades home on October 28, 2023. First responders — who arrived at the property following a cardiac arrest call — declared him dead at the scene.

While it was initially believed Matthew had drowned, the toxicology report confirmed he died from acute effects of ketamine.

Per the L.A. County Medical Examiner’s Office, Matthew had been undergoing ketamine infusion therapy for depression and anxiety more than a week before his death — but the M.E. concluded the ketamine in his system when he died wasn’t from the treatment.

Of course, that suggests Matt got ketamine from another source … possibly outside of a medical setting. That’s exactly what law enforcement has been trying to ascertain.

This wouldn’t be the first time the feds helped investigate a drug-related celeb death — they got involved in Mac Miller’s 2018 overdose as well — which resulted in 3 men being indicted for supplying the drugs that killed the rapper.

In May 2022, Stephen Walter was sentenced to more than 17 years in prison … after taking a plea deal for supplying Mac with fentanyl-laced pills.

Ryan Reavis got an 11-year sentence as the middleman who delivered the drugs from Walter to Cameron Pettit, who then gave them to Mac. Pettit’s case is still pending.

Expect similar charges if law enforcement ultimately succeeds in tracking down the source of Perry’s fatal dose of ketamine.