The Los Angeles Police Department, the Drug Enforcement Agency, and the U.S. Postal Service are working together to investigate the cause of Matthew Perry’s death , the L.

A. Times reported May 21. The Friends actor died of a ketamine overdose on October 28.

Now, a criminal investigation is being launched to figure out the source of the drugs. Other factors reportedly involved in Perry’s death included drowning in a hot tub, coronary-artery disease, and the effects of buprenorphine, a drug that treats opioid addiction and pain, though Perry had no other drugs in his system, according to a medical examiner. The amount of ketamine in his bloodstream was roughly equivalent to the amount used in general anesthesia, per the Times .

Perry was, at one point, on a regular schedule of ketamine infusions. However, it is reported that his last known infusion was over a week and a half before his death, and it wouldn’t have been enough to kill him..