Vermont health officials reported on Wednesday three deaths related to fentanyl, only a day after state police released a statement urging the public to steer clear of the extremely toxic drug that has so far been linked to numerous deaths across the Northeast.

Vermont Department of Health commissioner Dr. Harry Chen told reporters that the three Addison County, Vt. victims may well have been unaware that they were using the deadly drug. Fentanyl, which health officials say “can stop your breathing and kill,” is frequently packaged and sold as heroin. “Heroin users may not know they are shooting up fentanyl, or they may not know just how deadly it is,” he said, adding that the drug is about 50 times as powerful as heroin.

Though similar stories have surfaced across the region, it is still not yet clear where the drug is coming from, state police said in their report. Speaking to the VT Digger, Major Glenn Hall of the Vermont State Police confirmed that, while investigators are working closely with partners in neighboring states as well as the Drug Enforcement Agency (DEA), no source has been identified and no drug seizure has tested as fentanyl. “We don’t always know what’s in these heroin bags,” he said, echoing the concerns of the state’s health department. “The people that are buying this stuff don’t always know what it is as well.”

What Is Fentanyl?

Fentanyl, a drug commonly administered intravenously in hospitals as an analgesic, was first synthesized in Belgium in the late 1950s. According to the DEA, the drug has inspired several analogues, with the strongest being Carfetanil — a substance 10,000 times stronger than morphine that is typically used to subdue large animals. While numerous fatalities have been recorded in the past, Report Stirewalt, a spokesperson for the health department, said that past deaths have been very different from the ones that are being reported now.

“The fentanyl related fatalities in the past have been pharmaceutical grade prescription medications (mostly transdermal patches) that have either been accidental overdoses by the patient or that would have been diverted and abused,” he explained.

The Vermont cases recall a string of similar deaths in Pittsburgh, where 17 people overdosed last month on heroin that had been cut with fentanyl and packaged in bags labeled “Theraflu.” These cases also raised the possibility that the drug may have been involved in the death of actor Philip Seymour Hoffman, who was found dead with a syringe in his arm on Feb. 2. While preliminary testing have found no traces of an adulterant, investigators say further analysis is underway.