MINNEAPOLIS — Pills marked as hydrocodone that were seized from Paisley Park after Prince’s overdose death actually contained fentanyl, the powerful opioid that killed him, according to a source with knowledge of the investigation.

The musician, who weighed only 112 pounds when he died April 21, had so much of the drug in his system, autopsy results later showed, that it would have killed anyone, regardless of size, the source said.

Prince did not have a prescription for fentanyl, a synthetic opioid that has been described as 100 times more powerful than morphine, the source said.

Despite the finding, investigators still aren’t certain how Prince ingested the fentanyl. They are leaning toward the theory that he took the pills not knowing they contained the drug.

An autopsy report released in June by the Midwest Medical Examiner’s Office said Prince died from an accidental, self-administered overdose of fentanyl. But it did not indicate how he obtained the painkiller, nor did it list any other cause of death or “significant condition.”

Illicit fentanyl has traditionally been mixed with or sold as heroin. But the Drug Enforcement Administration said drug traffickers have since expanded the illicit fentanyl market by producing counterfeit pills that contain the opioid.

And while the 2006 raid of a Mexican drug lab halted an earlier surge in fentanyl-linked overdose deaths, authorities say China-sourced fentanyl and precursor chemicals are now being sold to criminals running clandestine pill-press operations across North America.

A recent flood of “wholesale amounts” of counterfeit pills that contain fentanyl prompted the DEA last month to issue a report warning of a rise in “overdoses, deaths and opiate-dependent individuals.” The DEA said it tested eight times as much fentanyl last year as it did during the 2006 crisis.

“This is becoming a trend,” according to the DEA’s report, “not a series of isolated incidents.”

Prince was found dead in an elevator at his Paisley Park compound in Chanhassen the morning of April 21, a day before he was to meet with a California doctor who specializes in opioid addiction. Two members of his inner circle found his body about 9:40 a.m.

A responding paramedic said Prince appeared to have been dead for at least six hours before his body was found.

Sources with knowledge of the investigation have said that autopsy results also revealed the presence of lidocaine, alprazolam and Percocet.

Prince died less than a week after an opioid overdose forced his private plane to make an emergency landing in Moline, Ill. He recovered after two shots of naloxone, an overdose antidote increasingly being used and often referred to by its brand name Narcan, a source said.