MINNEAPOLIS (AP) —  Federal authorities including the Drug Enforcement Administration are joining the investigation of Prince's death.

A statement Wednesday from the U.S. Attorney's Office says that office and the DEA are joining the Carver County Sheriff's Office in investigating Prince's April 21 death at age 57.

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. The same official also said investigators are looking at whether Prince had suffered an overdose when his plane made an emergency landing in Moline, Illinois, less than a week before he died.

Earlier Wednesday, a Minnesota attorney said the day before Prince died, his representatives reached out to a California doctor who specializes in addiction treatment to set up an urgent meeting with Prince.

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An expert on treating addiction has questioned whether a California doctor responded appropriately when he was called to help Prince.

Speaking without direct knowledge of Prince's case, addiction medicine expert Dr. Stuart Gitlow questioned why Dr. Howard Kornfeld sent his son, Andrew, on a flight to Prince's home in Minnesota rather than calling for emergency help.

The Kornfelds' lawyer, William Mauzy, said Wednesday that Prince's representatives told Howard Kornfeld that the singer was "dealing with a grave medical emergency" when they first contacted him the day before the musician's death.

Gitlow says if a doctor believes there is 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."

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A Minneapolis attorney says he believes Minnesota law would protect a California man from any potential charges related to Prince's death.

William Mauzy represents Dr. Howard Kornfeld and his son, Andrew. Mauzy says Andrew Kornfeld traveled to Minnesota to see Prince after his father was asked to help the singer deal with a painkiller addiction.

He says the younger Kornfeld showed up at Paisley Park on April 21 and was one of three people to find Prince unresponsive. The singer was soon pronounced dead.

Mauzy says Andrew Kornfeld, who isn't a doctor, was carrying a small amount of buprenorphine, which Kornfeld advocates for patients with addiction issues to provide pain relief with less possibility of overdose and addiction.

Andrew Kornfeld has been questioned by investigators, but Mauzy says he believes he is protected by a Minnesota law that lets people report overdoses without fear of prosecution. The attorney also says Andrew Kornfeld never intended to give the buprenorphine directly to Prince, but rather to a doctor who had planned to evaluate Prince.