The federal prosecutors who indicted Mayor Eric Adams on corruption charges last week told a judge on Thursday that at least part of their case against him involved classified material that might damage national security if disclosed.

In a letter, prosecutors requested a classified meeting in a secure courtroom with District Judge Dale E. Ho, without lawyers for Mr. Adams present, because “the government intends to discuss certain classified information,” the U.S. attorney, Damian Williams, wrote.

While it was unclear what information in the Adams case was so sensitive that prosecutors were seeking to keep it out of open court, such efforts to comply with the law known as the Classified Information Procedures Act did not necessarily indicate that the evidence against Mr. Adams was any more serious or damning than what was previously known, said Daniel Richman, a former federal prosecutor who now teaches at Columbia University.

“I don’t think this necessarily is an indication of anything beyond the fact that foreign actors were involved, and they may have been surveilled,” Mr. Richman said. “We cast our net pretty broadly when it comes to foreign agents.”

Alex Spiro, a lawyer for Mr. Adams, said in court on Wednesday that he was not concerned about any of the evidence prosecutors planned to disclose, be it classified or not.

“We don’t believe they have any evidence in this case at all to prove these charges,” he said.

Mr. Adams faces five federal charges of bribery, fraud and soliciting illegal foreign campaign donations, after an investigation that started in 2021, before he was elected mayor. Prosecutors say he received illegal campaign donations, discounted airfare and lavish hotel suites from Turkey in exchange for official actions, including using his influence to pressure the Fire Department to allow a new building that houses the Turkish Consulate to open despite problems with its fire safety system.

Mr. Adams has denied the charges and insisted that he will not resign. His lawyers filed a motion asking a judge to dismiss the case. Mr. Spiro has been unusually aggressive in the mayor’s defense, accusing prosecutors of misconduct by illegally leaking information about the case to reporters.