Pills labeled as hydrocodone that were seized from Prince's Paisley Park home after his death contained fentanyl, the drug that killed him, a source close to the investigation told the Minneapolis Star Tribune.
This new information gives authorities new insight into what led to Prince's overdose.
Prince did not have a prescription for fentanyl, so either the pharmaceutical manufacturer mislabeled the pills or Prince obtained illegally manufactured pills.
According to the Start Tribune, investigators are leaning toward the theory that the pop star didn't know the pills contained the synthetic opioid.
If the manufacturer had mislabeled the pill, there would have likely been a recall because fentanyl is 50 times more potent that heroin, according to CNN.
Autopsy results showed that at the time of his death on April 21, Prince had so much of the drug in his system that it would have killed anyone regardless of size, a source told the Star Tribune.
So what is fentanyl?
The synthetic drug is the most powerful painkiller on the market, often prescribed to cancer patients who are in extreme pain, according to CNN.
But fentanyl is illegally manufactured and readily available across the U.S.
According to the Star Tribune, the drug has traditionally been mixed or sold as heroin without the customer's knowledge. Counterfeit pills even caused overdose deaths in Minnesota and North Dakota earlier this year.
A Drug Enforcement Administration report published last month said counterfeit pills are fueling the fentanyl and opioid crisis.
Between late 2013 and 2014, there were more than 700 fentanyl-related deaths reported in the U.S.
The DEA expects that number to rise due to the expanding fentanyl market. According to the DEA, the drug is usually produced in overseas labs in China and marketed to drug trafficking groups in Mexico, Canada, and the United States.