As street drugs have become ever more powerful and deadly, a small nonprofit in Manhattan dedicated to preventing overdoses has drawn politicians and health officials from around the country searching for possible solutions to the opioid epidemic.

But now, a federal law enforcement official is warning that the group’s methods are illegal, and is suggesting that his office could crack down — and perhaps even end the effort.

The group’s strategy — known as supervised consumption — is simple, but radical: Let people use illicit drugs at special centers, under the watchful eye of nonprofit workers who can intervene if something goes wrong. The group, OnPoint NYC, said on Tuesday that its two overdose prevention centers had reversed 1,000 overdoses since they opened more than a year and a half ago, at a time when the drug supply contains the dangerous opioid fentanyl in growing concentrations.

The centers, in East Harlem and Washington Heights, were authorized by City Hall in 2021, and are the only such sites operating openly in the nation. Research on more than 100 such sites in other countries, where some have operated for decades, has found they reduce public drug use and lower mortality rates.

But on Monday night, in response to a request for comment, the U.S. attorney responsible for Manhattan issued an apparent warning that came in stark contrast to the more tolerant approach the Biden administration had been signaling toward the centers.

“I have repeatedly said that the opioid epidemic is a law enforcement crisis and a public health crisis,” Damian Williams, the U.S. attorney for the Southern District of New York, said in a statement to The New York Times. “But I am an enforcer, not a policymaker.” Until New York lawmakers take action to authorize the supervised consumption sites, he said, they are operating in violation of federal, state and local law.

“That is unacceptable,” he added. “My office is prepared to exercise all options — including enforcement — if this situation does not change in short order.”

The centers are considered illegal because the fentanyl, crack and other drugs consumed there are controlled substances, and because a federal law known as the “crack house statute” bars individuals from maintaining property where controlled substances are consumed.

In 2019, the Trump administration took legal action to block a similar center in Philadelphia from opening. The case remains in litigation. But OnPoint’s supervised consumption sites have functioned for 21 months without disruption by law enforcement and with the knowledge of local, state and federal authorities.

The U.S. attorney’s office did not immediately clarify what options it was considering, including whether it might move to shutter the centers if lawmakers did not act. A spokeswoman for the Department of Justice in Washington said that supervised consumption sites were being evaluated on a district by district basis, in discussion with local leaders, to determine “appropriate regulatory guardrails.”

There is visible drug activity on the block around OnPoint’s East 126th Street site, and detractors have criticized such centers generally, suggesting that they condone rather than deter drug use, potentially making the problem worse.

Sam Rivera, the executive director of OnPoint NYC, rejects those arguments. The area has been a “hot block” for drug dealing for years, he said, not because he brought it there, but because it made sense to open where drug users already are.

“It is really frustrating that a health issue has become a political issue,” he said in an interview at the center. “What we know is no one ever has to die again of an overdose, ever. They’re preventable.”

The main cause of the current overdose crisis is fentanyl — an opioid 50 to 100 times stronger than morphine — which has been detected in more than 80 percent of accidental drug overdose deaths in New York City. The city had 2,668 fatal drug overdoses in 2021, and 2022 is expected to be even worse once the official numbers are tallied. Statewide in 2021, 5,841 people died.

Gov. Kathy M. Hochul has declared the situation a public health emergency. The state has won $2.6 billion in settlements from opioid makers and distributors, and last fall, an advisory board recommended that she use some of the money to fund supervised consumption centers like OnPoint.

She declined, citing the legal barriers. Instead, this summer she announced plans to channel the funds into other harm reduction efforts, including fentanyl test-strip distribution, syringe exchange programs, drug treatment programs and an educational podcast about drug abuse called “Addiction: The Next Step.”

“Governor Hochul is committed to aggressively tackling the opioid crisis and has invested a historic $2.8 billion in addiction services since taking office,” said Avi Small, her spokesman.

Former Mayor Bill de Blasio authorized the supervised drug use centers through an executive action shortly before leaving office, and Mayor Eric Adams has continued to indicate his support and called for more centers to be established. But the supervised consumption component of their work is funded by private donors, often people who lost a loved one to an overdose, and not public funds, Mr. Rivera said.

Far from being in the shadows, the center hosts a steady stream of officials from other states and communities that are considering establishing their own sites. Four of the five local district attorneys in New York City — all but Staten Island’s — have said they back the centers.

The centers have street teams that clean up needles and encourage users to come inside, where they are also offered medical care, food, acupuncture, and case workers who can arrange for detox upon request. They would like to be would be open 24 hours a day, instead of roughly 12 hours a day on weekdays, Mr. Rivera said.

Other organizations that work to help drug users are seeking to open similar centers in the Bronx and other boroughs.

The National Institute on Drug Abuse, a federal entity, announced in May that it is funding a five-year study to assess the impact of OnPoint on drug users and the communities in which it operates. Rhode Island is the only state to have authorized such centers through legislative action, though none have yet opened there.

Among those who have had their overdoses reversed at OnPoint is Eddie M., 69, a former typesetter who travels almost every day to East Harlem from Brooklyn so he can inject his drugs under supervision. Because he uses illegal drugs, he asked that only the first initial of his last name be used.

Last Tuesday afternoon, he sat in a mirrored booth, so that staff members could monitor him, and prepared a speedball — a mixture of crack cocaine and street opioids — before loading it into a needle. A staff member tied off his upper arm with a blue tourniquet to help him find a vein.

Before discovering OnPoint, Eddie would use in hallways or parks. At OnPoint, there is a crash cart filled with equipment ready to go if he loses consciousness, which has happened to him there at least three times.

It’s been months since he overdosed, though, and he said the staff at the center have taught him to be more careful about how he uses. “The people are just wonderful,” he said. “I mean, I have never experienced anything like it.”

The centers have been used about 84,000 times by about 3,700 registered participants since opening, OnPoint said.

Participants bring in their own drugs, sign in and write down what they will be using, and can smoke or inject. Outside the supervised injection area, they can relax and watch television, or get other services.

It is impossible to know how many of the 1,000 overdoses OnPoint says it has reversed would have been fatal without intervention. Because they intervene so quickly, staff members are able to handle most overdoses simply by administering oxygen. When they do use naloxone, a drug that reverses the effect of opioids, they typically inject a small dose. They have called for an ambulance only about 20 times, said Rayce Samuelson, an OnPoint overdose prevention specialist.

In Albany, legislation has been introduced to more explicitly permit the centers and grant some legal immunity to those who operate them, but it has not passed. The City Council has also not passed legislation authorizing the centers.

Gustavo Rivera, a state senator from the Bronx who sponsored the Senate bill, has argued that Governor Hochul could expand state support to the centers through executive action alone. There are precedents, he said, such as her creation of a state agency to regulate marijuana despite its being illegal under federal law.

Ms. Hochul’s office noted that marijuana was legalized in the state before she funded that agency. In March, she said it was the city’s “prerogative” to experiment with the centers and even expand them, “but as far as a statewide policy, it’s something that we are exploring.”

“I would encourage us to be brave about this,” Senator Rivera said. He has been taking other lawmakers to tour the centers to try to convince them to vote yes. “As we do nothing, more and more people die in every corner of our state.”