Police plan to obtain Prince's medical records from North Memorial Clinic where Schulenberg worked. Andrew Kornfeld was carrying a small amount of the prescription drug buprenorphine. The sheriff's office said the detectives were "revisiting the scene at Paisley Park", CBS Minnesota reported.
The identity of a doctor who had been treating Prince several weeks before he died was revealed in a search warrant filed in Hennepin County District Court.
On the morning of Prince's death, Schulenberg had arrived at Paisley Park with test results for the singer.
The focus of the investigation into the death of Prince appeared to be shifting Wednesday to the role of a Minnesota doctor revealed in a search warrant to have seen the entertainer twice in the weeks before his death.
The warrant was carried out last Thursday at North Memorial Medical Center in the Minneapolis suburb of Robbinsdale. Schulenberg's LinkedIn profile has been made private.
"Once federal authorities like the DEA start asking questions, people get pretty anxious and go into hiding", a legal source said.
Meanwhile, police cars were seen back on Prince's estate on Tuesday afternoon.
California doctor Howard Kornfeld was first contacted by Prince's representatives on April 20, one day before the singer's body was discovered at his home, attorney William Mauzy told reporters.
The U.S. Drug Enforcement Administration and deputies from the local sheriff's department executed a search warrant on Tuesday at Prince's Paisley Park home as part of an ongoing investigation into the singer's death.
The Star Tribune reported that "the warrant does not describe the nature of the tests" whose results Schulenberg was planning to deliver that day. Nelson wrote in the affidavit that Schulenberg told him that he had treated Prince on April 7 and on April 20.
Johnson, Prince's friend, is mentioned in the search warrant documents as having been interviewed by investigators and telling them of medical treatment that Prince received for an unspecified illness in 2014 or 2015 at the Two Twelve Medical Center in Chaska.