A doctor did not prescribe the fentanyl Prince was killed by.

The 57-year-old musician passed away at his Paisley Park home in April after a self-administered overdose of the drug but, as it was not prescribed, TMZ reports it will be almost impossible for anyone to be charged with his death.

The website added that it is believed the pills - which were found in a bottle with the label hydrocodone - were obtained illegally.

If the person who gave Prince the drug is found, they could face a third-degree murder charge.

Meanwhile, Prince's friend and collaborator Judith Hill previously revealed the singer ''fought for his life'' after he took an overdose of Percocet on a flight only days before he passed away.

She recalled: ''We knew it was only a matter of time; we had to get down. We didn't have anything on the plane to help him. [When we arrived at the hospital], he was awake, which was such a relief to me, because I thought he was gone ... He wasn't dreary or drowsy, or anything. He wanted to watch 'Zootopia'. He loved those films. I was going to pull it up on my phone. He said, 'No, no, no, not here. We're going to pick a special time and place to watch that.' He was very cooperative that whole night, serious about getting help.

''He did it because he was concerned, and he wanted to do the right thing for his own body. And that's the part that breaks my heart, because he was trying. He was trying ... He told me, 'I had to fight for my life. I remember hearing your voices from afar and saying to myself, 'Follow the voices, follow the voices, get back in your body, you gotta to do this.' And he said it was the hardest thing he'd ever done, to get back into his body like that.''