Eric Adams, a retired police captain who was elected as New York City’s 110th mayor nearly three years ago on a promise to rein in crime, has been indicted in a federal corruption investigation, people with knowledge of the matter said.

The indictment remained sealed on Wednesday night, and it was unclear what charge or charges Mr. Adams will face. But the federal investigation has focused at least in part on whether Mr. Adams and his campaign conspired with the Turkish government to receive illegal foreign donations.

When the indictment is made public, Mr. Adams, a Democrat, will become the first New York City mayor to face a federal charge while in office.

The indictment promised to reverberate across the nation’s largest city and beyond, plunging Mr. Adams’s embattled administration further into chaos just months before he is set to face challengers in a hotly contested mayoral primary.

And, if it contains allegations of conspiring to commit crimes with foreign nationals, it will have landed on the same week that the city was playing host to leaders from across the world at the United Nations General Assembly, including Turkey’s president, Recep Tayyip Erdogan.

Mr. Adams struck a defiant tone in a video statement issued Wednesday, insisting that he had done nothing wrong.

“I always knew that if I stood my ground for New Yorkers that I would be a target — and a target I became,” he said. “If I am charged, I am innocent, and I will fight this with every ounce of my strength and spirit.”

Brendan R. McGuire, who with his partner at WilmerHale, Boyd M. Johnson III, represents the mayor, said the lawyers had no comment.

Representatives of the agencies that conducted the investigation into Mr. Adams — the U.S. attorney’s office for the Southern District of New York, the F.B.I. and the city’s Department of Investigation — declined to comment.

Even before he was indicted, Mr. Adams’s administration had been battered not just by the investigation into him and his campaign but by three separate inquiries involving some of his highest ranking aides and advisers — investigations that included a drumbeat of searches and seizures that destabilized City Hall and made it difficult for him to govern effectively.

Calls for Mr. Adams’s resignation had been steadily accumulating over the past several weeks.

Earlier on Wednesday, Representative Alexandria Ocasio-Cortez of New York said she could “not see how Mayor Adams can continue” in his job.

On Wednesday night, the news of the indictment fueled the criticism. Scott Stringer, the former New York City comptroller who is among the Democrats running against Mr. Adams in next year’s mayoral primary, said that the mayor was presiding over “a broken-down train wreck of a municipal government” and that he should step down.

The indictment represented an extraordinary turnabout for Mr. Adams, 64, a former state senator and Brooklyn borough president who took office as the city was rebounding from the pandemic and about to confront a massive influx of migrants from the southern border.

It grew out of an investigation by the F.B.I. and federal prosecutors in Manhattan that began in 2021 and was focused at least in part on the possible foreign donations, and on whether Mr. Adams pressured officials in the Fire Department to sign off on the opening of a new high-rise consulate building for the Turkish government despite safety concerns.

The investigators were also examining whether Mr. Adams accepted pricey flights and upgrades on Turkish Airlines, which is partly owned by the Turkish government. And they sought information about a Brooklyn construction company run by Turkish Americans, and a small university in Washington, D.C., with Turkish ties.

Mr. Adams has said he has visited Turkey at least six times and that he met Mr. Erdogan when Mr. Adams was Brooklyn borough president.

The inquiry remained secret until late last year, when an F.B.I. search of his chief fund-raiser’s home thrust it into public view. After searching the home of the fund-raiser, Brianna Suggs, last November, federal investigators left with two laptop computers, three iPhones and a manila folder labeled “Eric Adams.” Ms. Suggs has not been accused of wrongdoing.

Days later, in a dramatic scene on a Greenwich Village street, F.B.I. agents told the mayor’s security detail to step aside, climbed into his S.U.V. with him and seized his electronic devices.

On the day of the search at Ms. Suggs’s home, agents also searched the New Jersey homes of Rana Abbasova, the director of protocol in the Mayor’s Office for International Affairs and Mr. Adams’s former liaison to the Turkish community, and Cenk Öcal, a former Turkish Airlines executive and member of the mayor’s transition team. Neither Ms. Abbasova nor Mr. Öcal has been publicly accused of wrongdoing.

But the investigation into Mr. Adams has already affected the careers of Ms. Suggs and Ms. Abbasova. Ms. Suggs left her position as the mayor’s fund-raising chief, he said after her home was searched, and City Hall placed Ms. Abbasova on leave after discovering she had “acted improperly,” according to a spokesman for the mayor.

Weeks later, Ms. Abbasova turned against the mayor, and she has been cooperating with the investigation. Her lawyer, Rachel Maimin, a former federal corruption prosecutor who is a partner at Lowenstein Sandler, could not immediately be reached for comment.

Until federal investigators closed in on him, Mr. Adams’s life had seemed a classic New York success story.

Raised by a working-class mother in Brooklyn and Queens, he overcame dyslexia and run-ins with the police, and then joined the Police Department himself.

During his time with the department, he rose to lead two groups that represented Black officers, the Guardians Association and 100 Blacks in Law Enforcement Who Care.

In those roles, he often criticized police leaders for their policies and their treatment of Black officers, but nonetheless advanced to the rank of captain by passing promotional exams.

Mr. Adams was investigated by the department four times, including for associating with criminals, one of whom was the boxer Mike Tyson, who was convicted of rape. Mr. Adams has said he did nothing wrong and cast the scrutiny as retaliation for his activism.

His first foray into politics, a run for Congress at 33, did not go far. A decade later, in 2006, he won a State Senate seat, beginning a seven-year tenure that included three re-election victories.

In 2010, New York’s inspector general found that Mr. Adams and other Senate Democrats had fraternized with lobbyists and accepted significant campaign contributions from people affiliated with contenders for a video lottery contract at Aqueduct Racetrack. The findings were referred to federal prosecutors, but no charges were brought.

During his fourth Senate term in 2013, Mr. Adams successfully ran for Brooklyn borough president, an office he would use as a jumping-off point for his bid for City Hall. Early in his tenure in Brooklyn, he organized an event to raise money for a new nonprofit, One Brooklyn, which had yet to register with the state. The invitation list was based on donor rolls for nonprofits run by his predecessor, records show.

A city Department of Investigation inquiry concluded that Mr. Adams and One Brooklyn appeared to have solicited money improperly from groups that either had or would soon have matters pending before the borough president’s office.

Mr. Adams’s aides emphasized to investigators that the slip-ups had occurred early in his administration, and promised to comply with the law in the future.

For years during his political career, Mr. Adams dreamed of becoming New York’s mayor, an ambition he realized by embracing diverse constituencies across the city, and an accomplishment he has said was divinely ordained.

As mayor, Mr. Adams vowed to return “swagger” to a city still emerging from the pandemic, and he surrounded himself in City Hall with friends and associates whose loyalty to him sometimes exceeded their policy expertise. Several had troubled pasts.