The year since Prince’s death has been filled with tributes and fans mourning the legendary musician. But despite the dedications to his memory, a megastar-studded memorial concert, and anniversary events surrounding him planned for later this month, a police investigation into the circumstances surrounding his passing has continued.

Newly released documents revealed that prescription medications in pharmaceutical bottles and labels in the name of one of Prince’s associates were found by investigators, but there were none under the musician’s name. The Minneapolis Star-Tribune reported that no evidence of the drug fentanyl, the substance that took Prince’s life, was found on the premises of his Paisley Park estate in suburban Minneapolis.

Prince succumbed last April 21 to an accidental overdose of the drug, medical examiners said last June. Law enforcement officials in Carver County, Minn., launched a probe into the death and executed 11 search warrants between the day of his death and Sept. 19, 2016. Those warrants, unsealed on Monday, revealed that he was never prescribed fentanyl but it is unclear how he obtained the painkilling drug.

According to the search warrant affidavits, Kirk Johnson, Prince’s drummer and business associate and Meron Bekure, his personal assistant, who found his body in an elevator, told authorities that he had been suffering with opioid dependency and withdrawal issues. A pamphlet on getting getting over drug abuse was also found at the house.

Prince was scheduled to meet with Andrew Kornfeld, who had come to Minneapolis to assess him for a drug dependency program that was headed by his father, Dr. Howard Kornfeld. Another doctor, Michael Todd Schulenberg, told detectives in the investigation that he had treated Prince twice on April 7 and 20, 2016, a prescription for the opiate drug oxycodone, but under Johnson’s name in order to protect his privacy.

The Star Tribune reported that since Prince’s death, a joint state and federal criminal investigation has been taking place and put the focus on the musician’s use of painkillers and through what methods he got them. Searches conducted at Paisley Park turned up bottles of various pills, some labeled as hydrocodone, the affidavits say. But they were found inside of other containers like vitamin bottles throughout the complex. Prince never had a prescription for fentanyl, which is considered highly potent and addictive, but some of the pills tested positive for a synthetic version of the drug.

According to the paper, investigators have been looking at the possibility that Prince may have taken the drug without knowing it. His autopsy revealed a lethal amount of fentanyl in his body, officials revealed last June.