Sources say the pills are commonly used to treat pain, and investigators have brought in the U.S. Drug Enforcement Administration to help with the case.
Investigators believe a health scare about a week before Prince’s death, which caused an unscheduled landing of his plane in Illinois, was likely the result of a reaction to the pain medication.
The pilot told air traffic control that a passenger, later known to be Prince, was unresponsive.
Prince was treated for a potential overdose of pain medication. His publicist later said he returned home and was fine.
Prince was 57 when he died last week at his home in a Minneapolis suburb.
Investigators are still awaiting results of autopsy and toxicology tests for further understanding of why Prince died.