Law Enforcement Investigating Matthew Perry Death, Source of Ketamine

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…