The political drama around the federal courthouse in Manhattan did not end with the sudden, unexplained turnabout of President Trump in firing Preet Bharara, the United States attorney for the Southern District of New York.

While that prominent post always gets attention, one candidate frequently mentioned as a possible successor to Mr. Bharara could sharpen the scrutiny to new levels: Marc L. Mukasey, a former prosecutor who now works in white-collar criminal defense.

As it happens, Mr. Mukasey has represented Roger E. Ailes, the former chairman of Fox News, who has long had a mogul-to-mogul relationship with Mr. Trump.

Whomever Mr. Trump nominates to replace Mr. Bharara will inherit an investigation of Fox News.

A federal grand jury sitting in Manhattan is expected to soon hear testimony from at least two witnesses about business practices at Fox News when it was led by Mr. Ailes, according to two people with knowledge of the matter. Mr. Ailes, who was forced out in July amid revelations of multiple accusations of sexual harassment, has denied those charges.