LONDON ¨C Music superstar Prince¡¯s autopsy found the painkiller Percocet in his system, the Minneapolis Star Tribune and KSTP-TV reported on Thursday, citing sources close to the investigation.

Prince also had a dangerously low red blood cell count, indicating he had been ill, Minneapolis ABC affiliate KSTP-TV said, citing two unnamed law enforcement officials.

A spokeswoman for the local medical examiner¡¯s office that conducted a post-mortem examination of Prince declined to confirm the reports.

The cause of Prince¡¯s death remained undetermined. The medical examiner¡¯s office said in late April the autopsy and toxicology results could take weeks.

The news reports came after federal authorities said on Wednesday they were joining the investigation into Prince¡¯s death.

The Drug Enforcement Administration and U.S. Attorney¡¯s office in Minnesota will bring federal resources to the local investigation and expertise on the illegal use and trafficking of prescription drugs, the U.S. Attorney¡¯s office said.

Also on Wednesday, a lawyer for a California addiction doctor said Prince¡¯s representatives had contacted the doctor the evening before his death, adding that the doctor had planned to visit Prince for a ¡°life-saving mission.¡±

The 57-year-old Prince was found dead on April 21 at his Paisley Park home-studio complex in a Minneapolis suburb. Prescription opioid medication was found at the scene, a law enforcement source said.

Dr. Howard Kornfeld, who runs Recovery Without Walls, a clinic in Mill Valley, California, planned to fly to Minnesota on April 22, his lawyer William Mauzy said. In the meantime, his son, Andrew Kornfeld, a clinic staff member, traveled to Minnesota on April 21 for an initial discussion.

When Andrew Kornfeld arrived at Paisley Park, Prince was not available, Mauzy said. A staff member found the artist unconscious in an elevator, and Kornfeld called 911.