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.