Massive Jackson Investigation into Rx

Dr. Conrad Murray has emerged as the primary target in the death of Michael Jackson -- but law enforcement sources tell us a massive investigation is taking form into what could be fraudulent prescription practices of more than a dozen doctors ... and we're told that number could easily double.

LAPD detectives and members of the L.A. County Coroner's office are gathering info on various doctors who treated Michael Jackson over the years. Subpoenas have been issued, medical files have been taken and conversations with various doctors and their lawyers are taking place.

Jackson has used more than 2 dozen aliases to score drugs, with the help of doctors ... 19 alone are mentioned in the affidavits used to secure search warrants against Dr. Murray.

We're told the prescription practices became so egregious that one prominent pharmacy wouldn't even bother to write the patient's name when a certain doctor called in scripts for Jackson.

Sources say law enforcement is looking at one doctor who used the same anesthesiologist dozens of times to put Jackson under for routine procedures that should not have required sedation.

Law enforcement sources say the Jackson case could be the biggest of its kind when it comes to doctors who enabled celebrity patients to fuel their drug addiction ... and we're told criminal charges could loom large.