In a signal of the widening investigation into political fundraising during Mayor Adams’ 2021 campaign, FBI agents quietly seized phones and an iPad from Mr. Adams early this week, his attorney disclosed Friday.

The seizures happened as Mr. Adams was leaving a public event in Manhattan, according to a statement from the mayor’s attorney, Boyd Johnson.

“On Monday night, the FBI approached the mayor after an event. The Mayor immediately complied with the FBI’s request and provided them with electronic devices,” Mr. Johnson said. “The mayor has not been accused of any wrongdoing and continues to cooperate with the investigation.”

The seizure of the devices, first reported by the New York Times, came four days after federal agents searched the Brooklyn home of Mr. Adams’ top campaign fundraiser, Brianna Suggs.

That search prompted the mayor to cancel a planned trip to meet with White House officials in Washington and instead return to New York.

In a statement on Friday, Mr. Adams, a former police captain, said he had “nothing to hide.”

“As a former member of law enforcement, I expect all members of my staff to follow the law and fully cooperate with any sort of investigation — and I will continue to do exactly that,” he said.

The seizure is the clearest evidence yet that federal investigators are interested in Mr. Adams, who has previously kept an arms-length distance from ethics scandals that have dogged several of his associates.

Mr. Adams, a Democrat, said nothing publicly about his phones being seized when he met with reporters on Wednesday and insisted he wasn’t aware of any wrongdoing by members of his campaign team. He said he would be “shocked” if anyone on his campaign acted inappropriately.

But in his statement Friday, Mr. Adams’ attorney said they had “discovered that an individual had recently acted improperly.” His campaign spokesperson declined to identify the person involved or say what they did wrong.

“After learning of the federal investigation, it was discovered that an individual had recently acted improperly. In the spirit of transparency and cooperation, this behavior was immediately and proactively reported to investigators,” Mr. Johnson said, offering no further details.

Representatives for the U.S. attorney in Manhattan and the FBI both declined to comment.

The focus of the investigation hasn’t been publicly disclosed by prosecutors, but a search warrant obtained by the Times indicated authorities are examining whether the Adams campaign conspired with the Turkish government to receive donations from foreign sources, funneled through straw donors.

The warrant also requested information about Mr. Adams’ use of New York City’s matching funds program, which provides candidates with an eightfold match of a city resident’s first donations.

Over the summer, Manhattan prosecutors brought charges against six people who allegedly sought to manipulate the fundraising program to funnel tens of thousands of dollars to Adams’ 2021 mayoral campaign. Adams has not been charged in that case.

A campaign spokesperson said the FBI has since returned some, but not all, of Adams’ devices.

Photos show Mr. Adams spoke Monday evening at an autism awareness event at New York University. The following day, his spokesperson announced that Mr. Adams would not travel to Puerto Rico for an annual conference widely attended by New York’s political players.

At his news conference Wednesday, Mr. Adams told reporters his absence at the conference was the result of New York’s “serious fiscal crisis,” adding that it wouldn’t be appropriate “to have on a flowery shirt lying on a beach drinking a margarita.” He was previously scheduled to receive a humanitarian award at the conference, according to a flyer of the event.

The Democrat also reiterated earlier statements that he had no personal knowledge of any improper fundraising, and didn’t believe he had anything to personally fear from the investigation. He praised Ms. Suggs, 25, as a “brilliant young lady” who “followed the rules.”

Ms. Suggs has declined comment through a spokesperson.

At the time, the mayor’s chief City Hall lawyer had confirmed that the administration was communicating with federal prosecutors, but declined to discuss what that entailed.

Mr. Adams, 63, was elected mayor two years ago and has been in office since January 2022. He went into politics after a 22-year police career, serving as a state senator and in the largely ceremonial position of Brooklyn borough president before running for mayor.

A political centrist, Mr. Adams rode a tough-on-crime message to victory in the 2021 Democratic mayoral primary before defeating Republican Curtis Sliwa in the general election.

In September, Mr. Adams’ top building-safety official, Eric Ulrich, was charged with taking $150,000 in bribes and improper gifts from associates and repaying them with favors that included access to top officials, including Mr. Adams, and help doing business with the city.