Matthew Perry's Cause of Death Still a Mystery, Autopsy Pending

Matthew Perry's death will remain a mystery for now -- because officials are waiting for some key tests to be done before weighing in definitively on how he died.

The Los Angeles County Medical Examiner's Office just published their findings on an initial autopsy of the late "Friends" actor -- and they're deferring his cause of death ... pending toxicology results and further investigation, which could take weeks to complete.

They're also not indicating the manner of death at this point ... although our sources have told us no foul play is suspected, and no illicit narcotics were found on the premises.

The only other notable detail from his autopsy report is that they note he died at his residence -- and that his body is now ready to be released to his family for burial/cremation. A probe into the circumstances of his death is underway with law enforcement.

TMZ broke the news ... first responders rushed to his property on a cardiac arrest call, and Matthew was found unresponsive in his hot tub ... this after an apparent drowning. The exact sequence of events is unclear, but he was eventually declared deceased on the scene.

Like we told you ... MP had played pickleball Saturday morning for a couple hours before returning home and sending his assistant on an errand. Sometime later, the assistant returned, discovered his boss ... and then called 911.

It goes without saying ... Matthew's passing sent shockwaves throughout Hollywood. Considering his history with substance abuse, there's been speculation that could be at play -- but so far, there's no indication that's in fact the case.

Medical professionals are going to wait to see what was in his system at the time of his death -- in the meantime, all his fans and loved ones can do is mourn.

No word yet on funeral plans ... but we imagine a major celebration of life is coming soon.