Federal agents are assisting authorities in Minnesota to determine how Prince obtained the fentanyl that killed him, whether through an illicit channel or from a doctor, according to a law enforcement official.
The official told the Los Angeles Times that there does not appear to have been a legitimate reason for Prince to have been using such a powerful painkiller. A spokesman for the Carver County Sheriff's office said Friday that "this investigation remains active" but was unable to provide specifics.
Russell Baer, a spokesman for the Drug Enforcement Administration, declined to comment on the investigation.
On Thursday, the Midwest Medical Examiners Office in Minnesota said that Prince died of a self-administered accidental overdose of fentanyl, a synthetic opioid that doctors often prescribed for patients dealing with chronic pain from late-stage cancer and is also used as an anesthetic during heart surgery.
In media interviews, close associates of Prince have said the performer was constantly in pain from years of athletic performances on stage.
The cause of death report didn't say how the fentanyl was administered or if there were any other substances in the pop star's system.
Those convicted of trafficking in drugs that result in someone's death or injury can face a stiff federal penalty, including jail time. But the presence of other substances in a deceased person's system can complicate a legal case.
Prince was found unresponsive in the early hours of April 21 at his home, Paisley Park, in Chanhassen, Minn. He was later declared dead.
Among those at the scene was Andrew Kornfeld, the son of Dr. Howard Kornfeld, a California physician who specializes in opioid addiction recovery.
The Kornfelds' attorney, William Mauzy, told reporters following Prince's April 21 death that Andrew had flown to Minneapolis in hopes of encouraging Prince to check himself into his father's Mill Valley rehabilitation program and that the buprenorphine he was carrying was intended to be turned over to a Minneapolis physician to be administered to the late pop star. According to Mauzy, the medication was not administered and was later taken into possession by sheriff's investigators.
On Friday, Andrew Kornfeld wrote an opinion piece for CNN in which he described the use of buprenorphine as a way of treating opioid addiction. Dr. Kornfeld and his son have declined to comment when reached by The Times.
He wrote on Friday that when a user relapses after going "cold turkey," and then takes the same dose that he previously tolerated, "that dose may prove fatal." Taking buprenorphine, he wrote, helps to eliminate the craving for opioids, as well as the symptoms of withdrawal.
The interaction of buprenorphine and fentanyl is potentially dangerous, according to State University of New York at Buffalo professor Richard Blondell, whose research focuses on addictions. But he said the research on the potentially dangerous interaction remains hazy.
According to an affidavit for a search warrant obtained by The Times, Prince saw a Minneapolis-area doctor, Michael Todd Schulenberg, the day before he died, and Schulenberg was at Paisley Park to deliver medical test results on the morning the musician was found dead.
The doctor, who specializes in family medicine, had also seen Prince earlier in the month, on April 7, and told investigators he had prescribed medication. The singer was supposed to fill the prescription at Walgreens, although it is unclear from the warrant whether he did.
It remains unclear what the prescription was for. A lawyer for Schulenberg said Friday she was unable to comment on the investigation.
3:17 p.m.: The story was updated with a comment from a DEA spokesman.