Toxicology tests have come to an official conlusion about what led to the death of pop superstar Prince, who passed on April 21 at the age of 57.

Prince died from an accidental overdose of the opioid fentanyl, the Midwest Medical Examiner’s Office has concluded, according to a CNN report.

Fentanyl is typically prescribed by doctors for treament of cancer, but is often used illicitly like many opioids. It is 25 to 50 time more potent than heroin, and 50 to 100 tiems more powerful than morphine.

Prince was found unresponsive in an elevator at his home and recording studio Paisley Park in Minnesota.

Authorities believe Prince self-administered the fentanyl. They believe the overdose was accidental. There is no indication of the fentanyl was prescribed or obtained illicitly.

Prince has been rumored to be a prescription drug abuser, according to CNN. Prince’s half-siblings have accused him of being addicted to Percocet for decades.

Prince’s plane made an emergency landing on April 15 on his way home from Atlanta after a performance. He was unresponsive and taken to the hospital.

“Pharmaceutical fentanyl is a synthetic opioid pain reliever, approved for treating severe pain, typically advanced cancer pain.1 It is 50 to 100 times more potent than morphine,” according to a CDC statement. “It is prescribed in the form of transdermal patches or lozenges and can be diverted for misuse and abuse in the United States. However, most recent cases of fentanyl-related harm, overdose, and death in the U.S. are linked to illegally made fentanyl.2 It is sold through illegal drug markets for its heroin-like effect. It is often mixed with heroin and/or cocaine as a combination product—with or without the user’s knowledge—to increase its euphoric effects.”