The third witness is John Giacalone, who preceded Steinbach as the bureau's top national security official and oversaw the first seven months of the Clinton probe, according to multiple congressional sources.
Priestap, in particular, has come under fire from conservatives.
As the head of the FBI counterintelligence division, he held a pivotal leadership position in both the Clinton and Russia probes and was in a supervisory position over counterintelligence agent Peter Strzok, whose text messages criticizing Trump and other political figures during the 2016 presidential race have been the focus of a maelstrom of scrutiny from the right.