WASHINGTON — In the office of Attorney General Jeff Sessions hangs a portrait of his predecessor Edwin Meese III, the Reagan-era conservative. Near the desk of the deputy attorney general is a painting of former Attorney General John Ashcroft, who pressed to expand the Justice Department’s powers after Sept. 11.

The department’s inspector general, Michael E. Horowitz, chose for his office a portrait of the disgraced Attorney General Harry M. Daugherty, who was nearly impeached, forced to resign in 1924 and later tried twice on charges of defrauding the government.

Mr. Daugherty’s visage serves as a reminder for Mr. Horowitz: Malfeasance is never far away.

Equal parts auditor, investigator and cop, Mr. Horowitz has navigated his role as one of the most powerful arbiters of conflict in Washington — the investigators’ investigator — with a diplomat’s instinct for recognizing fault lines, a prosecutor’s focus on justice and a Washington insider’s knack for amassing allies on Capitol Hill.

Now, he has been thrust into the clash of law enforcement and politics that has consumed much of the capital for the past two years. On Thursday, he will issue the highly anticipated findings of his examination of the F.B.I.’s handling of its investigation into Hillary Clinton’s use of a private email server. He is expected to castigate the decision making by the former F.B.I. director James B. Comey; his deputy, Andrew G. McCabe; and former Attorney General Loretta Lynch.