(CNN) Pills seized inside Prince's Paisley Park compound by investigators were labeled as hydrocodone but actually contained fentanyl -- the drug that killed the singer -- according to a source with knowledge of the investigation who revealed the information to the Minneapolis Star Tribune.
The revelation gives more insight into what may have caused Prince's overdose. There are two likely possibilities: Either a pharmaceutical manufacturer mislabeled the pills, or the pills were illegally manufactured and obtained illegally.
However, according to the Star Tribune, investigators are working under the theory that the pop star did not know the pills contained fentanyl. Prince died on April 21 from an accidental overdose of fentanyl, according to the medical examiner.
If the manufacturer mislabeled the pills, there would have likely been a recall, as fentanyl is an extremely potent opioid medication and a controlled substance. It would be a serious health hazard if a batch was mislabeled and given to the public.
Fentanyl is 50 times more potent than heroin and up to 100 times more potent than morphine.
Investigators have not said whether Prince ingested those pills or whether he knew the pills contained fentanyl.
Star Tribune sources said Prince's toxicology report, which has not been released, also revealed the presence of lidocaine, alprazolam and Percocet in his system.
CNN's Dr. Drew Pinsky said the combination of fentanyl, (which is an opioid based drug), and alprazolam (which is a benzodiazepine) could be the key to why Prince died.
"It is all too common for people to overdose if they are taking a combination of a benzodiazepine and an opioid," Pinsky said. "This is what I've been warning was a likely possibility."
At the beginning of the investigation into Prince's death, sources told CNN that painkillers were found inside his Paisley Park compound and they could not find a valid prescription for the medication. The Star Tribune now reports that their source said no prescription for fentanyl has been found in the months since Prince's death. So, a burning question remains. How did Prince acquire the drug?
Fentanyl, the most powerful painkiller on the market, is normally prescribed to cancer patients in extreme pain. It's also used to ease the pain of someone who is dying or as part of anesthesia during surgery.
But illegally manufactured fentanyl pills are readily available across America.
Just last month, the Drug Enforcement Administration published a report saying counterfeit pills are fueling the fentanyl and heroin/opioid crisis in the United States, which has become an epidemic. The DEA issued a national health alert over the rise in fentanyl overdoses in 2015.
That year, the DEA said its National Forensic Laboratory Information System showed "there were approximately eight times as many fentanyl exhibits (confiscated drugs that tested positive for fentanyl) in 2015 as there were during the 2006 fentanyl crisis, clearly demonstrating the unprecedented threat and expansion of the fentanyl market."
Fentanyl, the new heroin -- but stronger
Prince's death at 57 years old shocked the world. For months fans mourned the musical genius by showing up by the hundreds to pay their respects. Many left purple mementos on the fence that surrounds the Paisley Park Compound where Prince died.
He was discovered dead inside of an elevator in the complex.
Officials never revealed the amount of fentanyl that was in Prince's system. But the Star Tribune reported that a source said the amount of fentanyl in his system was so high it would have killed anyone, no matter their size.
It has been four months since Prince died and the investigation into his death is still ongoing. The DEA began working the case alongside the Carver County Sheriff's Office from the very early stages. But no one knows exactly why Prince was taking strong painkillers or how he obtained them.