NEW YORK (AFP) - A specialist in painkiller addiction was rushing to see Prince before the music legend's death but arrived too late, a lawyer for the doctor said Wednesday.

Howard Kornfeld, who treats people with addictions to opioids, had been due to see Prince at his Minnesota compound on April 22, a day after the pop star died, his lawyer William Mauzy said.

"Dr Kornfeld felt that his mission was a life-saving mission. He certainly felt it to be urgent," Mauzy told reporters in Minneapolis.

Kornfeld runs the Recovery Without Walls clinic in Mill Valley, California, north of San Francisco, which offers confidential treatment for substance addiction as well as chronic pain.

"The hope was to get him (Prince) stabilised in Minnesota and convince him to come to Recovery Without Walls in Mill Valley," Mauzy said.

The doctor had cleared much of his schedule after being contacted by Prince's representatives and in the meantime had asked both his own son - a medical student who works with his father's clinic - and a Minnesota doctor to visit the often reclusive superstar.

But by the time the younger Kornfeld knocked on the door of the Paisley Park complex on April 21 and was let in by a Prince associate, the Purple Rain singer was unconscious, according to the lawyer.

The son, Andrew Kornfeld, was the voice heard in a 911 emergency call pleading for an ambulance, according to the lawyer. Kornfeld names the victim as Prince but is unable to give the street address of Paisley Park.

The account of the lawyer, who was confirming a report in the Star Tribune newspaper of Minneapolis, is the biggest signal yet that painkillers may have played a role in Prince's death.

Investigators conducted an autopsy before Prince's cremation but have not yet released a cause of death, although the Carver County Sheriff's Department said there was no sign of trauma to his body or evidence of suicide.

Prince, who was 57, had appeared vigorous at recent concerts and had long been legendary for his marathon concerts, often performing for hours with little break or putting on two shows per night.

But Prince went through a hip replacement surgery in 2010. His April 14 concert in Atlanta, which turned out to be his last, had been rescheduled after he cited the flu as a reason he could not make a previous date.

On his way back from Atlanta, Prince's plane made an unscheduled stop in Moline, Illinois.

He again brushed off his illness as the flu, inviting fans afterward to Paisley Park for a dance party, but the Star Tribune quoted anonymous sources as saying that the star had overdosed on opioids, which are generally used to treat severe pain.

The lawyer said that Andrew Kornfeld was questioned by authorities but allowed to return to California. He did not administer medicine to Prince or even see him alive, the lawyer said.

The 911 call was a "Good Samaritan" gesture that should provide him legal immunity, Mauzy said.