Matthew Perry's Death Being Investigated by United States Postal Service

Matthew Perry's death is mobilizing multiple federal agencies -- TMZ has learned the DEA's now getting assistance from the U.S. Postal Inspection Service in its efforts to find the person who gave the actor the ketamine that killed him.

Law enforcement sources tell us ... the USPS' federal investigatory authorities are now involved in the probe, and looking into whether Matthew acquired any drugs through the mail. We're told the postal inspectors are offering their expertise -- including their ability to track packages and other mail-related investigation tools.

TMZ broke the story ... local police and the DEA have been investigating the circumstances surrounding MP's passing for months -- after the toxicology report found the actor died from acute effects of ketamine.

Though the report noted Matthew had been undergoing ketamine infusion therapy for mental health issues more than a week before his passing, the medical examiner determined the ketamine in his system at the time of his death wasn't from the treatment ... raising the question of whether he got it from another, non-medical source.

Many of Matthew's close friends in Hollywood have said the actor was in a really good place before his untimely death ... with many believing he was sober and making plans to start a foundation to help others with addiction issues.

Matthew was found unresponsive in the hot tub at his Pacific Palisades home in October ... his assistant called 911, but first responders were unable to revive him. He was declared dead at the scene.