The US attorney's office and the Drug Enforcement Administration (DEA) will be helping on the case, Minnesota police have confirmed.

Jason Kamerud of the Carver Country sheriff's office told the Hollywood Reporter yesterday that “they have agreed to provide federal resources and expertise in our investigation.”

Prince passed away suddenly on 21 April at his Paisley Park estate in Chanhassen, Minneapolis.

Earlier this week, it came to light that Dr. Howard Kornfeld, an addiction specialist, was due to meet with Prince the day after his death to help deal "with a grave medical emergency". Kornfeld's son Andrew, who works with him, was set to visit Paisley Park on 21 April to discuss treatment with Prince; Andrew actually phoned 911 upon discovering Prince in an elevator.

Kornfeld's lawyer William Mauzy says that “The plan was to quickly evaluate his health and devise a treatment plan... the doctor was planning on a lifesaving mission."