Horowitz, the Department of Justice (DOJ) inspector general, is an increasingly critical player in the controversy surrounding the FBI, President Trump and the Russia investigation.

With little fanfare, he has been conducting a sprawling probe of the FBI's handling of the 2016 investigation into Hillary Clinton's use of a private email server. His full report, which could set off shockwaves, is expected by the early spring.

A political appointee in both the Bush and Obama administrations, Horowitz's yearlong investigation already reportedly contributed to the early resignation of Deputy FBI Director Andrew McCabe. And his work has been felt in other ways.

Horowitz also uncovered a series of text messages between FBI officials Peter Strzok and Lisa Page that led special counsel Robert Mueller to remove Strzok from his team. Those texts have fueled accusations among GOP lawmakers that Mueller's probe is tainted by partisanship.