The F.B.I. agent corps today overwhelmingly fits the demographic profile of a Trump voter. During the 2016 campaign, in The Guardian, one agent said, “The F.B.I. is Trumpland.” In his testimony, Mr. Strzok all but laughed out loud when committee members pressed him Thursday on whether the whole F.B.I. was made up of Democrats.

The New York field office, one of only three headed not by a special-agent-in-charge but by a full assistant director, has always been a particular challenge for bureau leaders — it’s fiercely independent, combative and notoriously leaky. The office, which works closely with the local United States attorney for the Southern District of New York, a job held by both Mr. Comey and Mr. Giuliani, is sometimes referred to inside the Justice Department as the “Sovereign District of New York” for charting its own course.

The office has long been a source of meddlesome leaks, in part because of the intermixing of F.B.I. agents and New York Police Department officers who have close relationships with the city’s press corps. The lowest point in these relations came in 2009, when the investigation of the would-be subway bomber Najibullah Zazi — a critical emergency investigation that had remained secret when it was focused in Denver, Zazi’s hometown — leaked quickly once the would-be attacker and case arrived in New York, both to the media and to the suspect’s family itself.