A man and a woman from Cincinnati were indicted Wednesday on charges of dealing heroin laced with carfentanil — the deadly animal tranquilizer officials blame for an unprecedented surge in overdoses in the United States — marking what may be the first carfentanil-related criminal case brought in federal court.

A grand jury charged Phillip Watkins, 31, and Jeannetta Crawford, 26, with distributing heroin cut with fentanyl and carfentanil, a heroin analogue that is 10,000 times stronger than the drug itself.

The indictment comes as overdoses have skyrocketed in Ohio and neighboring states, rattling law enforcement and health officials, who say heroin tainted with carfentanil has sent people to the emergency room in record numbers in the past two months.

Prosecutors called the case a new front in their crackdown on the drug.

“This is the first carfentanil trafficking case that we have ever brought in this district, in the region, in Ohio,” acting U. S. attorney Benjamin Glassman told reporters Wednesday, according to Cincinnati.com. “And I believe it is the first federal carfentanil indictment ever in the country.”

Carfentanil, typically used to sedate elephants and other large animals, is the most powerful commercial opioid in the world, and authorities say dealers have been cutting it into their heroin supplies to give users longer, more potent highs. The drug is odorless and colorless, so it’s nearly impossible for users to know what they’re taking. A single grain of it is enough to kill a person.

Authorities said carfentanil began turning up in the Cincinnati area, as well as other parts of the state, in mid-July. In the weeks that followed, health officials in three states had linked carfentanil to hundreds of overdoses, dozens of them fatal. In a six-day period late last month, 174 people across Cincinnati were hospitalized for overdosing on heroin that was likely cut with the drug — a number the county health commissioner called “unprecedented.”

“It’s unlike anything we’ve seen before,” Hamilton County Commissioner Dennis Deters told the Cincinnati Enquirer at the time.

Carfentanil may have also been the culprit in a widely publicized overdose in East Liverpool, Ohio, earlier this month. On a weekday afternoon, police stopped a vehicle for driving erratically and found two unconscious adults inside, along with a 4-year-old boy. Someone took a picture of the man and woman, blue-faced and slumped over in the front of the car, with the confused-looking boy sitting behind them in a car seat. The city posted the image on Facebook and it was later shared more than 28,000 times.

The seven-count indictment against Watkins and Crawford, said to be boyfriend and girlfriend, alleges they ran a drug operation out of their apartment in northern Cincinnati and were caught selling tainted heroin on at least four occasions.

An affidavit in the U.S. District Court for the Southern District of Ohio says that police first learned about the couple in late August, after they found a person overdosing in the driver’s seat of a car in nearby Springfield Township, Ohio. They used the anti-overdose drug Narcan to revive the victim, who said Watkins and Crawford sold the heroin that triggered the overdose.

The victim, who is not identified in court documents, agreed to serve as a confidential informant for investigators, saying two other people had also overdosed on the drugs allegedly sold by the couple. On several occasions in August, the person went to the couple’s apartment and bought drugs, secretly recording the exchanges with a hidden audio wire, according to the affidavit. Investigators said they also tapped phone calls to the alleged dealers.

Lab tests of the drugs came back positive for fentanyl, carfentanil and heroin, court documents said.

On Aug. 31, a SWAT team raided the couple’s apartment and arrested Watkins and Crawford, according to the affidavit.

After they were released, investigators said, Watkins got a new cellphone and continued to deal, and police arranged two more controlled buys. Officers arrested them again on Sept. 15, according to WCPO.

They were charged with narcotics conspiracy, operating a drug premises, and distribution of a controlled substance and a controlled substance analogue.

Attorneys for Watkins and Crawford didn’t immediately respond to a request for comment Thursday.