One of the clues is a special logo on the illegally distributed stuff.
The other clue they found recently is the name of a lab owner who produces and distributes illegal drugs; he is known as Dr. Azar among drug dealers.
Don’t miss the fourth episode of the series tonight at 19:00 GMT.
Each episode of the seven-part series is also repeated the next day at 03:00, 08:00, and 14:00.
The series, produced by Saeed Parvini and directed by Mehrdad Khoshbakht, is about four policemen and their involvement in city operations.
It deals with a number of social issues, including addictive drugs.
The series' cast includes Bahram Ebrahimi-Yazdi, Reza Karimi, Amir-Reza Delavari, Farhad Qaemian, Parisa Moqtadi, Niloufar Shahidi, Asghar Heidari, and the director.