Prince: Overprescribing Drugs ... Focus of Investigation

Authorities in Minnesota have made the overprescribing practices of Prince's doctors the focus of their investigation ... sources connected with the case tell TMZ.

We're told the criminal probe has zeroed in on a drug that almost killed Prince 6 days before his death -- Percocet. TMZ broke the story, EMTs gave Prince a "save shot" at the Moline airport after his plane made an emergency landing.

We're told law enforcement in Minnesota is looking at 3 possible abuses ... multiple doctors who overprescribed meds to the singer, aliases the singer and his doctors may have used, and multiple pharmacies that filled the prescriptions.

As we reported, Prince had prescriptions filled at least 4 times in the 7 days before he died, and at least one of the pharmacies was 6.1 miles from his home ... much farther than 8 other drug stores.