(NBC News/Alex Johnson) — The drug that killed Prince is a narcotic so powerful that it was blamed for the accidental deaths of as many as 120 hostages when Russian forces used it to subdue Chechen terrorists during the 2002 siege of a packed Moscow theater.

A medical examiner's report Thursday said Prince — who was found dead April 21 in an elevator at his home studio in Chanhassen, Minnesota — died from fentanyl toxicity.

The autopsy report from the Midwest Medical Examiner's Office in Ramsey, Minnesota, gave no details apart from declaring that Prince, 57, died from an apparently self-administered dose of fentanyl. It's still not known how he got the drug or, if it was prescribed, why.

But at the time of his death, Prince was being treated for opioid withdrawal, as well as anemia and a fatigue, a source with knowledge of his treatment told NBC News.

Meanwhile, local investigators have asked for help from the U.S. Drug Enforcement Administration, federal law enforcement officials have told NBC News.

"This was a man in his 50s who may have been struggling with pain and took a very potent opioid analgesic and died accidentally from an overdose," Barbarajean Magnani, pathologist-in-chief at Tufts University Medical Center of Boston, told NBC station KARE of Minneapolis on Thursday.

"Celebrities bring it to our attention, but we see this every day," Magnani said. "We have to re-examine the way we're treating pain."

Fentanyl is a synthetic opioid — its chemical name is the tongue-twisting N-phenyl-N-[1-(2 phenylethyl)-4-piperidinyl] monohydrochloride — that was first formulated during the 1950s as a safer and more effective alternative to the painkillers morphine and meperidine.

Its creators at the Belgian drug company Janssen Pharmaceutica got the "more effective" part right.

Fentanyl is the strongest opioid approved for medical use in the United States, rated as 50 to 100 times more potent than morphine and 30 to 50 times more potent than heroin, according to the National Institute for Drug Abuse.

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