Prince: Criminal Investigation Full Bore ... Doctors On The Hot Seat

The Prince criminal death investigation is in high gear, and authorities are scrutinizing two doctors ... law enforcement sources tell TMZ.

Our sources say authorities are taking a hard look at Prince's primary physician Dr. Michael Schulenberg and renowned addiction specialist Dr. Howard Kornfeld, to determine if either of them crossed the legal line by writing prescriptions to Prince or otherwise facilitating the singer's acquisition of meds.

We're told three agencies are deep into the investigation -- the FBI, the DEA and the Carver County District Attorney's Office.

Prince died after OD'ing on Fentanyl, an extremely powerful narcotic. TMZ broke various stories that the singer was addicted to Percocet and almost died from the drug 6 days before his ultimate passing.

We also broke the story ... hours before Prince's death, he was photographed at a Walgreens near his home in Paisley Park pacing and looking anxious as someone went inside to get a prescription.

Dr. Schulenberg was at Prince's home when his body was discovered, but says he was only there to inform Prince of some test results. Dr. Kornfeld's son was also at the house with a backpack loaded with synthetic opiates to treat addiction. Kornfeld's lawyer says none of the opiates went into Prince's system.

As for the two doctors, Kornfeld is still actively practicing in Marin County, CA but Dr. Schulenberg has been MIA from his med clinic since Prince died in April.

Our sources say if state charges are filed, the case will not go to the grand jury ... the DA will make the decision. We're told it's unclear how long the investigation will take, but it could be several months.