New York City Mayor Bill de Blasio met with federal corruption prosecutors for four-plus hours Friday in his lawyer’s midtown Manhattan law office for an unusual voluntary grilling in which he agreed to answer questions related to a long-running criminal probe of his fundraising.

De Blasio, who for months has faced a swirl of questions about possible favors to donors, including disgraced Long Island restaurateur Harendra Singh, spoke to prosecutors without a grant of immunity, according to City Hall, and didn’t talk to reporters staking out the session when he left.

“We remain confident that at all times the mayor and his staff acted appropriately and well within the law,” de Blasio press secretary Eric Phillips said in a statement Friday afternoon. “We hope our continued cooperation will help bring a swift conclusion to the U.S. Attorney’s review.”

A federal contingent from Manhattan U.S. Attorney Preet Bharara’s office seen arriving at the building at 6th Avenue and 46th Street on Friday morning around the time of de Blasio’s arrival included Andrew Goldstein, head of the public corruption unit, his deputy Tatiana Martins, and prosecutor Russell Capone.

Goldstein and Martins were on the trial teams that successfully prosecuted former Assembly Speaker Sheldon Silver and former Senate Majority Leader Dean Skelos. Bharara’s office and the FBI declined to comment on the meeting Friday, or even confirm that it happened.

News that the meeting was happening triggered a media frenzy outside the offices of defense lawyer Barry Berke. When de Blasio arrived in the back of a police-driven black SUV just after 9 a.m. and pulled into a loading dock, his vehicle was swarmed by reporters and cameras. By the time he left, just after 1:45 p.m., police had set up barricades, but reporters chased the SUV as it departed.

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Federal and state probes of de Blasio over the past year have targeted various aspects of his fundraising. Singh, a donor from Laurel Hollow, rented a restaurant property from the city, and investigators have explored whether the administration tried to intervene on his behalf in a dispute about the lease.

Singh was also indicted on Long Island in a corruption case involving Nassau County Executive Edward Mangano, and has been negotiating cooperation deals there and in Manhattan. The federal probe of de Blasio has also reportedly included matters ranging from City Hall’s role in a controversial real estate deal to its stance on carriage horses and a trash-bag contract.

In another sit-down in December, confirmed by City Hall, de Blasio met with investigators from the Manhattan district attorney’s office to answer questions relating to a grand jury probe of whether his political team broke the law by funneling donations to county committees in an effort to flip control of the State Senate to Democrats.

Meetings with the government by potential targets without immunity are unusual and risky — prosecutors could, for example, pursue false statements or anticipate a trial defense.

But legal experts say a politician may feel he needs to appear to have nothing to hide, and de Blasio’s willingness to sit down probably also indicates that his lawyers believe evidence is not definitive and that he is likely to avoid charges.

“I respect defense counsel in this case,” said veteran Manhattan criminal lawyer Gerald Lefcourt, an acquaintance of Berke, “and feel pretty sure he wouldn’t permit his client to be questioned in this way unless he felt pretty confident that this will be the end of it and there will be no prosecution.”

After Friday’s session with the government, de Blasio was scheduled to fly to Georgia to attend a Democratic National Committee meeting to choose a new chairman. De Blasio is supporting Minnesota congressman Keith Ellison.