Federal prosecutors have issued a grand jury subpoena demanding testimony from Daniel C. Richman, a close confidant of James B. Comey, the former F.

B. I. director and one of President Trump's most reviled political adversaries. The subpoena, issued by prosecutors in the U.S. attorney's office for the Southern District of Florida, appears connected to the so-called grand conspiracy case seeking to implicate Mr. Comey and other federal officials in an overarching plot to deprive President Trump of his rights.

It remains unclear what testimony prosecutors in Florida want from Mr. Richman, a law professor at Columbia University who once served as an adviser to Mr. Comey at the F. B. I.

But he has found himself caught up in two previous investigations of his friend and former boss. One inquiry sought to determine whether Mr. Comey had used Mr. Richman as a conduit to leak information to the news media about sensitive investigations. The other resulted in an indictment last September accusing Mr. Comey of lying to Congress about some of the same matters covered in the leak investigation.

The investigation is being led by Joseph di Genova, a Trump loyalist who has convened a grand jury in the Federal District Court in Fort Pierce overseen by Judge Aileen M. Cannon. The grand conspiracy case is arguably the most far-fetched of the Justice Department's serial efforts to pursue Mr. Trump's political opponents.