An autopsy scheduled for today will help determine what caused the star to collapse in an elevator at his beloved Paisley Park estate.
Though a report in TMZ claims that Prince had been hospitalized for a drug overdose less than a week before his death, a source who worked with him a few years ago believes he was grappling with a serious and ongoing illness at the time.
"The people close to him were very concerned for his health and indicated he'd been undergoing treatments which made his immune system weak," the source tells PEOPLE.
Spike Lee Hosts Street Party To Celebrate Prince
The Purple Rain icon's death comes a week after his private plane made an emergency landing, and the star was rushed to a hospital in Moline, Illinois, but released three hours later. His rep told PEOPLE at the time that the star was still fighting a prolonged battle with the flu.
Although the privacy-loving star generally kept mum about his personal life, collaborator and friend L.A. Reid said told the Today show on Friday that his pal was "clean" and a committed vegan.
"The thing that really bothers me about it is the Prince I know was super-healthy, vegan, wasn't an abuser of drugs, wasn't an abuser of alcohol," he said. "He was clean and he looked young and he looked really healthy and vibrant so the whole thing is really mysterious to me."