Authorities could be closer than ever to discovering what killed iconic musician Prince. Investigation into his mysterious April 21 death has led police to the early conclusion that “a fatal mix of drugs” may have been at work.

According to a report from TMZ, Prince may have had both Percocet and Buprenorphine in his system. At present authorities believe the combination may be responsible for killing the “Purple Rain” singer, though they are still waiting on toxicology reports from the medical examiner for confirmation. Now the question remains: If Prince did take the fatal combination of drugs, how did they get into his body? Those assigned to the investigation have a renewed interest in Dr. Michael Schulenberg, Dr. Howard Kornfeld and his son Andrew Kornfeld.

Schulenberg was Prince’s doctor. Police began targeting him in their investigation May 10 after he revealed to them he had written prescriptions for Prince that were filled at a Walgreens pharmacy he frequented. Prince is said to have been at the store four times in the week before his death, though it is unclear whether the prescriptions he was filling were all written by Schulenberg and for what. Percocet, for which Prince is rumored to have been filling scripts, is a controlled substance. Federal regulations would not have allowed the doctor to write four prescriptions for the painkiller in one week. On May 11 — the same day police obtained warrants to seize documents and other materials linked to the physician — it was reported the doctor had abruptly left his practice in Minnetonka, Minnesota. Sources close to him did not know if he was fired or quit.

As for the Kornfelds, Prince is thought to have reached out to Howard Kornfeld about addiction treatment on his own volition. A source told the the New York Times the singer’s call was prompted by the concern of family and friends.

An opioid addiction specialist at Recovery Without Walls, Howard Kornfeld is said to have spoken to Prince several times prior to his death. The pair arranged to meet on the morning of his death, but Prince did not show. As such, the doctor sent his son to Prince's home, Paisley Park in Chanhassen, Minnesota. Andrew Kornfeld — who is not licensed to practice medicine — found Prince’s body inside. A lawyer for the Kornfeld family said Andrew Kornfeld, who was with two other people, was in the pop icon’s home only to explain his father’s practice. He is said to have been carrying Suboxone — a synthetic opiate — but did not administer any to Prince prior to his death and had no intention of doing so. His lawyer claims Andrew Kornfeld was simply transporting the drug for his father, bringing it to another doctor.

While Prince’s lawyer, L. Londell McMillan, maintains that he lived a clean and healthy lifestyle, friends of the Grammy award winner suggest otherwise. According to a May 12 report from Us Weekly, Prince was battling a secret addiction to Percocet that stemmed from nearly one decade ago. One friend, Sky Dangcil, said the singer began using the painkiller after hurting his hip. He is thought to have used the pills to “keep the show going” despite the pain he was in. Dangcil, who played music with Prince for seven years until 2007, said the “When Doves Cry” singer refused to admit he was struggling.

“He was always ‘on,’ so he wouldn’t complain or say anything was wrong,” Dangcil said at the time.

Another close friend echoed Dangcil’s statements. Kim Berry, who was Prince’s hair stylist, told the publication she speculated the legendary musician’s many sleepless nights, coupled with years of “jumping off speakers with 4-inch shoes on,” probably contributed to his aches and pains. She said Prince refused to slow his pace for one second.

The Midwest Medical Examiner’s Office in Ramsey, Minnesota, has yet to announce Prince’s official cause of death. In a May 5 statement the office provided a few brief details about his death but said it wanted to wait until it was absolutely certain to make an official ruling. The investigation into Prince’s death is said to be “ongoing.”