Not long before Prince died of an accidental fentanyl overdose, federal authorities sounded the alarm of a new flood of “wholesale amounts” of counterfeit pills laced with the deadly synthetic opioid.

The substance’s path from far-flung labs in Mexico and China to U.S. drug markets, and users often unaware that the illicit pills contain the highly toxic substance, make it difficult for investigators to track down the source of the drugs — even in the highest-profile of cases.

On Thursday, Carver County Attorney Mark Metz said evidence suggested that Prince unknowingly took counterfeit Vicodin painkillers that actually contained fentanyl. Prince ingested so much of the powerful drug before his April 21, 2016, death that the dosage would have killed anyone, regardless of body weight or tolerance, authorities have said.

Metz also said that investigators do not believe that anyone associated with Prince knew the pills stamped to appear as an “exact imitation” of Vicodin actually contained fentanyl — which can be up to 50 times more potent than heroin.

Metz said law enforcement uncovered evidence that Prince also likely took the counterfeit Vicodin before he passed out while on a flight from Atlanta to Minneapolis in the days before he died, requiring his private plane to make an emergency landing in Moline, Ill.

It appears Prince became another casualty to a worrying trend.

“In many cases, the colorings, markings, and shape of the counterfeit pills were consistent with authentic prescription medications,” according to the U.S. Drug Enforcement Administration’s most recent drug threat assessment. “The presence of fentanyl may only be determined during laboratory analysis.”

Drug traffickers in recent years have begun using fentanyl powder and pill presses to pump out pills resembling popular prescription opioids like oxycodone and hydrocodone, the DEA said.

Investigators found dozens of counterfeit pills in Prince’s Paisley Park dressing room and in over-the-counter pill bottles on his nightstand. But they haven’t been able to trace how the drugs got to Prince. Metz said Thursday that law enforcement combed through Prince’s computer and “all digital evidence” of the notoriously private musician in an attempt to find out how he got the pills. He said investigators were hamstrung by the fact that Prince didn’t own a cellphone, which might have recorded the phone numbers or texts used to buy the painkillers.

The DEA said that all of the black market fentanyl has so far been linked to producers in China or Mexico.

“China-sourced fentanyl concealed in mail parcels can be difficult for law enforcement officials to trace back to the original sender due to the use of freight forwarders,” according to the DEA National Drug Threat Assessment. “The original supplier in China will provide the package to a freight forwarding company or individual, who transfers it to another freight forwarder, who then takes custody and presents the package to customs for export. Additionally, these packages are often incorrectly manifested to avoid law enforcement detection. The combination of a chain of freight forwarders and multiple transferals of custody makes it difficult for law enforcement to track these packages.”

Fentanyl, or related compounds, are also sold through illicit drug markets on the so-called dark web. The DEA said this allows customers to use conceal their identity through Web browsers to buy the substances with untraceable digital currency and have them shipped directly to their homes or other mail drops.