Calling the recent surge in opioid-related overdoses one of “the biggest public health and law-enforcement crises of our time,” prosecutors in Brooklyn announced the indictment on Wednesday of 34 people charged with running a sprawling drug ring that sold a potent new designer narcotic never before seen in New York City — furanyl fentanyl.

Furanyl fentanyl — or White China, as it is sometimes known — is a chemical analogue of fentanyl, a synthetic painkiller that can be as much 50 times as powerful as heroin and has been blamed in some areas of the country for killing more people than heroin itself. Prosecutors said that the furanyl fentanyl seized in Wednesday’s case was produced in China, shipped to the United States by private mail and sold for as little as $7 to $10 per dose by a network of distributors in all five boroughs of New York.

“These synthetics are lethal, they are cheap and they continue to flood into our streets,” Eric Gonzalez, the acting Brooklyn district attorney, said at a news conference detailing the charges. Mr. Gonzalez added that the monthslong investigation of the drug ring, nicknamed Operation Hardball, turned up evidence that the traffickers were earning as much as $1 million a year.

Though furanyl fentanyl was made illegal by the federal government in November, it is a new enough phenomenon that it has not yet been classified as a controlled substance under New York State law. As with K2, a synthetic form of marijuana that wreaked havoc across the city this summer, producers of fentanyl and its analogues can sometimes avoid prosecution by tweaking the chemical recipe of the drug to work around specific provisions of the law.