The painkiller Percocet was present in Prince's body when he was found dead April 21 in a Paisley Park elevator, a source familiar with the investigation said Wednesday.

However, it is not yet clear whether the potent opioid caused or contributed to the musician's death, the source stressed.

The revelation came on the same day that federal law enforcement authorities joined the investigation into the death of Prince, who was seeking emergency help for an addiction to opioid painkillers during the week of his collapse.

The Drug Enforcement Agency (DEA) and the U.S. attorney's office announced that they were assisting Carver County investigators following the public disclosure that a California physician had been called for addiction treatment. The doctor was scheduled to meet Prince in the Twin Cities the day after the artist was found dead.

Criminal investigators are scrutinizing how Prince acquired the painkillers, or any medications used to treat the addiction.

A spokesman for the U.S. attorney's office declined to comment Wednesday, other than to say that federal officials can advise Carver County investigators regarding the diversion of prescription drugs beyond their prescribed uses. Confronting the diversion of opioid pain medications has been a priority for the DEA, the Department of Justice and U.S. Attorney Andrew Luger.

"While this remains an ongoing investigation, we will have no further comment," the spokesman said Wednesday.

Prince was pronounced dead at 10:07 a.m. April 21, 19 minutes after emergency responders arrived after receiving a 911 call. The cause of death remains undetermined, pending the results of an autopsy and toxicology tests. While those tests have not been completed, they showed that Percocet was present, the source said Wednesday night.

Among the first to discover Prince was Andrew Kornfeld, who had flown overnight from California at the request of his father, Dr. Howard Kornfeld, a specialist in addiction treatment. According to William Mauzy, a prominent Minneapolis-based attorney working with the Kornfelds, Andrew Kornfeld was supposed to talk to Prince about returning to California for extensive treatment. He also brought a starter dose of buprenorphine, a drug commonly prescribed to treat opioid addiction. Howard Kornfeld was scheduled to meet with Prince a day later.

A few days earlier, on April 15, Prince suffered an opioid overdose and his plane was diverted to Moline, Ill., while en route from Atlanta to Minneapolis, according to sources with knowledge of the investigation.

Medics in Moline gave him a dose of naloxone, or Narcan, to counteract the opioids.

Less than a week later, representatives for the musician reached out to Howard Kornfeld's Recovery Without Walls clinic in Mill Valley, Calif., which specializes in the use of Suboxone, a drug containing buprenorphine, as part of addiction treatment.

When Prince started taking opioid medications is unclear, but news reports indicated he underwent hip surgery several years ago due to the pounding his body took from his energetic stage performances.

An increasing body of research supports the effectiveness of Suboxone, which contains buprenorphine to manage pain and naloxone to reduce opioid cravings.

The famed Hazelden Betty Ford Foundation, based in Center City, Minn., has long favored abstinence and counseling for treatment, and surprised many in 2013 when it added Suboxone to its opioid addiction program.

Since that time, the number of opioid addicts dropping out of treatment early has declined from 25 percent to 5 percent, said Dr. Marvin Seppala, Hazelden's chief medical officer.

"It limits cravings and helps a number of patients with opioid-use disorder engage in treatment and stick around in treatment," he said.

Suboxone, however, is surprisingly hard to access in Minnesota, where only about 120 doctors have completed the federal certification necessary to prescribe it.

Extra training and supervision is needed, in part because Suboxone has a street value and can be misused by addicts to manage their cravings until they can afford more heroin or prescription opioids.

Seppala has found it difficult to refer Hazelden patients to doctors closer to their homes who can prescribe the medication. An organization called Heroin Project Ltd. was formed by a retired Burnsville physician specifically to recruit more doctors to provide the drug.

"Primary-care doctors … don't like treating people with addiction," Seppala said. "They don't want them in their waiting rooms. They feel like it prevents other people from coming to see them. I think that's primarily based on stigma and bias."

Representatives for Prince called Howard Kornfeld the night of April 20, because the musician "was dealing with a grave medical emergency," Mauzy said. They reached out to the doctor, he said, because of his national reputation as an addiction researcher.

As a licensed California doctor with a DEA registry to prescribe controlled substances, Howard Kornfeld would have been able to issue medications to a Minnesotan. He has published research in medical journals regarding buprenorphine and has been quoted in multiple news reports based on his expertise with the medication.

The Kornfelds provided authorities with information, along with the initial dose of medication that was intended for Prince, Mauzy said.

While Carver County authorities have characterized the case as a criminal investigation, that doesn't mean that it will result in charges.

Results from the death investigation could still be weeks away, due in part to the complexity of interpreting toxicology tests and determining whether any drugs found in Prince's bloodstream were unusual and toxic enough to have contributed to his death. 612-673-7744 612-673-4465