Investigators are increasingly focused on a drug overdose as the likely cause of Prince's death, and are reportedly seeking evidence of any crime linked to the singer's drug supply.

The county sheriff's office in Prince's suburban Minneapolis community revealed that the investigation had taken a twist when it refused to release more information on the case, citing legal authority to suppress details related to the "criminal" probe, reported the Minneapolis Star-Tribune.

"Investigative data collected or created by a law enforcement agency in order to prepare a case against a person, whether known or unknown, for the commission of a crime ... are confidential ... while the investigation is active," said a report citing the law by the Carver County Sheriff's Office.

As part of the probe, investigators are seeking the source of the prescription pain pills found on the 57-year-old musician and at the scene when Prince was found unresponsive in his Paisley Park Minnesota home the day he died, the newspaper reported.

They are also attempting to determine if a doctor prescribed any drugs that may have been linked to the death — or if there was a physician on board when the singer made an emergency landing for treatment of a drug overdose as he was flying home from a concert in early April.

Prince was reportedly unconscious when his plane landed in Illinois, and paramedics had to revive the singer, who was given a "save shot" antidote to pull him out of an overdose on an undisclosed opioid drug.

A Minnesota judge has sealed a search warrant in the investigation of Prince's death after authorities argued that making it public would hamper their work. The search warrant is for Paisley Park, the musician's home and recording studio complex in suburban Minneapolis.

Prince died at the complex on 21 April, and an initial search was carried out that day. It could be several weeks before toxicology tests are competed as part of Prince's autopsy.