A search warrant was obtained in relation to the home of Prince shortly after he died, court documents show.
The musician's Minneapolis home and recording studio, Paisley Park, was searched on 21 April, the day he died.
But authorities won a court order on Thursday to keep the warrant sealed, arguing that making it public would hamper their work.
Prescription painkillers were in the singer's possession following his sudden death, officials told US media.
But it is unclear what role, if any, those drugs may have played in his death.
Carver County Deputy Sheriff Jason Kamerud said reports that the Drugs Enforcement Agency (DEA) had been called in were not accurate.
A post-mortem last week discounted suicide - but a medical examiner said full results could take several weeks.
Prince, 57, was found dead in a lift on his Paisley Park estate.
There was no sign of trauma on the body, Sheriff Jim Olson said, suggesting the singer had died alone.
There have been suggestions the star was addicted to the painkiller Percocet, which he had been taking since 2009 to relieve hip pain.
His former percussionist, Sheila E, told a news agency Prince had suffered the effects of years of jumping off speakers on stage while wearing high heels.
However, claims of drug dependency have been refuted by the singer's long-time lawyer, L. Londell McMillan.
Although Mr McMillan admitted that Prince may have taken pills to manage pain, he said the musician was "not on any drugs that would be any cause for concern".
Prince had been rushed to hospital in Illinois six days prior to his death, while flying home from a concert in Georgia, but was treated and released a few hours later.