On the eve of former special counsel Robert S. Mueller III’s long-awaited congressional testimony, President Trump vented his fury on Twitter — but not at Mueller.

Trump’s ire on Tuesday night was reserved for Aaron Zebley, a top Mueller aide added at the 11th hour by Democratic lawmakers as a sworn witness in Wednesday’s hearing before the House Intelligence Committee and as an adviser to Mueller in his House Judiciary Committee testimony.

Former special counsel Robert S. Mueller III testified before the House Judiciary and Intelligence Committees on July 24. (The Washington Post)

Calling Zebley a “Never Trumper lawyer” involved in the “Crooked Hillary case,” the president termed his last-minute inclusion in the hearing a “disgrace.” “This should NOT be allowed,” Trump tweeted late on Tuesday. “Rigged Witch Hunt!”

The president’s outrage continued early on Wednesday, as he argued in another tweet that he “NEVER would have agreed” to Zebley’s role in the hearings.

If Mueller — a rigidly impartial lawman and longtime registered Republican — was always an awkward target for Trump’s anger, Zebley is an even more unlikely character to face the brunt of presidential indignation.

Once a globe-trotting special agent, he spent years hunting terrorists before becoming a federal prosecutor and then Mueller’s chief of staff at the FBI. He’s also apolitical, despite Trump’s claims, with no donations to either party, according to federal records. But his role during a stint in private practice representing a former Hillary Clinton aide during the FBI probe into her use of a private email server earned him a starring, if unnamed, role in Trump’s Twitter feed.

A graduate of the College of William & Mary and the University of Virginia School of Law, Zebley was assigned as a young agent to the FBI’s I-49 squad, a group then devoted to hunting down the bombers who attacked U.S. embassies in Kenya and Tanzania in 1998, according to author Garrett M. Graff’s book “The Threat Matrix: The FBI at War in the Age of Global Terror.”

Zebley showed his operational mettle in 1999 while tracking a bombing suspect to South Africa, Graff writes. When he got to Cape Town, he realized the man he was hunting wasn’t there after all — but quickly learned that one of the nation’s most wanted terrorists, Khalfan Khamis Mohamed, was living as a refugee in town. So Zebley and his partner staked out a center where refugees got their documents stamped until Mohamed showed up, then baited him into a government building before tackling him and shipping him back to the U.S. for trial. He was later sentenced to life in prison.

After years on the terrorism beat, on Sept. 10, 2001, Zebley transferred to a criminal investigation division — a reassignment that lasted exactly one day, until terrorists struck the World Trade Center and Pentagon. The veteran agent soon became a key player in PENTTBOM, the FBI’s massive probe into the 9/11 attacks, toiling in the basement of the J. Edgar Hoover Building. Zebley helped build the case against 9/11 plotter Zacarias Moussaoui and took the stand during the death-penalty phase of his trial in 2006 to testify about the agency’s work.

After a stint as an assistant U.S. attorney in Northern Virginia, Zebley served as Mueller’s chief of staff while he was FBI director. In 2014, he followed his boss into private practice in Washington as a partner at WilmerHale, where he helped Mueller on notable cases, including a probe into the NFL’s handling of star running back Ray Rice’s domestic violence case.

In 2017, he left behind a $1.4 million gig at WilmerHale to join Mueller’s probe into Russian influence in the 2016 election. Along with Mueller’s deputy, James L. Quarles III, and another former prosecutor, Michael Dreeben, he was among the inner circle directing the investigation.

Soon after his appointment to the team, some conservatives zeroed in on one of Zebley’s private practice cases as they made the argument that Mueller’s operation was slanted against the president.

They noted that Zebley had represented Justin Cooper, an ex-adviser to former president Bill Clinton who helped Hillary Clinton set up the private email server that would later become the center of a federal probe. Although Cooper later testified to Congress, earning plaudits from Republican Rep. Jason Chaffetz, then the chairman of the House Oversight Committee, for “having the guts to actually answer questions,” commentators such as Fox News host Tucker Carlson argued that Zebley’s work suggested a bias against Trump.

That line of attack reemerged on Tuesday, when Democrats announced that they had agreed to allow Zebley a role in Wednesday’s hearings.

Mueller had requested that Zebley be allowed to testify at both committee hearings, The Washington Post’s Rachael Bade and Matt Zapotosky reported, but the Judiciary Committee denied that request. Instead, Zebley will sit next to the former special counsel and advise him on his answers. Zebley will be sworn in to the Intelligence Committee.

Republicans cried foul about the surprise move.

"The chairman’s unprecedented decision to allow a witness’s counsel to both advise him privately and simultaneously testify alongside him shows the lengths Democrats will go to protect a one-sided narrative from a thorough examination by committee Republicans,” said Rep. Douglas A. Collins of Georgia, the Judiciary Committee’s top Republican.

Other elected Republicans quickly noted Zebley’s link to the Clinton email server case.

Trump, too, slammed the news, suggesting that Zebley’s inclusion was an “unfair” trick by Democrats.

Mueller’s office, though, says that Zebley is there simply to aid Mueller — the same role he’s played for years, dating back to their work at the FBI. Indeed, in the lead-up to his testimony, The Post reported, Mueller’s former chief of staff has been holed up in a law office helping him prepare for the testimony.

“Aaron Zebley was the Deputy Special Counsel and had day-to-day oversight of the investigations conducted by the Office,” Jim Popkin, a Mueller spokesman, said in a statement. “He will accompany Special Counsel Mueller to the Wednesday hearings, as was discussed with the committees more than a week ago.”

More from Morning Mix: