Federal prosecutors who brought corruption charges against Mayor Eric Adams last fall have recently presented additional evidence to a grand jury in his case, suggesting that they are taking steps to seek more charges — either against him, other people or both.

The disclosure that prosecutors have presented more evidence came from an unusual source: the mayor’s defense lawyers, who revealed in court papers dated Dec. 24 that a new witness had recently testified before the grand jury, whose proceedings are conducted in secret.

Days after the prosecutors announced the indictment of the mayor on Sept. 26, they said it was possible that they would bring additional charges. But the new testimony is the first public sign in three months that they are taking steps to do so, a prospect that some legal experts said had begun to seem increasingly unlikely. The presentation of additional evidence to the grand jury, which was confirmed by two people who were told of it, has not been previously reported.

The five-count indictment against the mayor charges him with bribery, fraud, soliciting illegal foreign campaign contributions and conspiracy as part of a scheme involving the Turkish government. He has pleaded not guilty and maintained his innocence.

Prosecutors said on Monday in an unrelated court filing in the mayor’s case that they have continued to “uncover additional criminal conduct by Adams” and “identify additional individuals involved,” another indication new charges may be in the works. The filing did not elaborate on the conduct they said they had discovered.

The possibility of new charges comes as the mayor is facing trial in April, just weeks ahead of a June Democratic primary in which he hopes to emerge yet again as the party’s mayoral candidate, despite the challenges facing his campaign.