Michael K. Williams' death of an apparent drug overdose has triggered a criminal investigation ... because law enforcement sources tell TMZ cops are trying to locate the drug dealer.
Our law enforcement sources say NYPD detectives are conducting an investigation into the origin of the narcotics cops found at the site of the actor's death over the weekend ... which we're told are believed to be heroin-based.
As we reported ... Michael was found dead in his Brooklyn apartment, and authorities found drug paraphernalia. An official autopsy has yet to be conducted ... but we're told drugs seem to be at play. A toxicology report should provide answers, but that could be weeks away.
A relative found Michael after not hearing from him for a few days ... and once he was discovered, a swarm of NY officials descended on his Brooklyn high-rise.
Michael, of course, had a prolific career ... and might perhaps be best known for his award-winning role as Omar in "The Wire." He also starred in "Boardwalk Empire," "12 Years a Slave" and "Lovecraft County." He was working up until recently ... with a few projects in the can, and a handful of others released in the past year or so.