MILL VALLEY, Calif. — The Northern California doctor who was asked to help Prince before his death is an addiction and pain specialist who has championed the use of a semi-synthetic opiate to treat pain.

Dr. Howard Kornfeld, who operates an outpatient medical center in bucolic Marin County, received publicity in the San Francisco Bay Area in 2013 for his work with buprenorphine.

Advocates of the drug say the opiate can help addicted patients by offering pain relief with less possibility of overdose and addiction, unlike oxycodone or morphine.

Representatives of Prince reached out to Kornfeld the day before the pop star died, but the doctor sent his son because he couldn’t immediately fly to Minnesota, said William Mauzy, a Minneapolis lawyer for Kornfeld.

The son, Andrew Kornfeld, was one of three people who found the singer unresponsive and called 911. He is listed as a consultant with his father’s clinic and is a pre-med student, Mauzy said.

A law enforcement official briefed on the investigation has told The Associated Press that investigators are looking into whether Prince died from an overdose. The official spoke on condition of anonymity because he wasn’t authorized to talk about the investigation.

At least two experts on treating addiction have questioned whether Kornfeld responded appropriately when he was called to help Prince. Dr. Stuart Gitlow, who has no direct knowledge of Prince’s case, said that if Kornfeld believed it to be a medical emergency, “his obligation is to call an ambulance and get the patient to emergency personnel who can assess the situation — not to fly to the patient.”

Howard Kornfeld told the Marin Independent Journal in 2013 that he first became aware of buprenorphine in the 1990s while treating heroin and prescription-drug addicts.

“We know this drug has no toxicity to the kidneys and much less toxicity to the liver than any other opiates. It works better for chronic nerve pain, and it doesn’t cause overdoses,” he said in an interview with the San Francisco Chronicle in 2013.