Robert Mueller's top deputy Aaron Zebley will be by his side during congressional hearings

Jonathan Ernst/Reuters

Former special counsel Robert Mueller sits with prosecutor Aaron Zebley as he testifies before the House Judiciary Committee at a hearing on the Office of Special Counsel's investigation into Russian Interference in the 2016 Presidential Election on Capitol Hill in Washington, DC, on July 24, 2019.

• The former special counsel Robert Mueller's top deputy, Aaron Zebley, was at his side on Wednesday as he testified before Congress.
• Zebley has been Mueller's right hand man for years and played a central role in the special counsel's Russia investigation.
• President Donald Trump aimed some of his Twitter outrage at Zebley ahead of the hearings.
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Aaron Zebley has been Robert Mueller's right hand man for years, and continued that role on Wednesday as the former special counsel faced questions from lawmakers in Washington.

Zebley, 49, played a key role in Mueller's investigation into Russian election interference and left a $1.4 million job at the WilmerHale law firm to become one of the office's first hires.

Trump attacked Zebley with unsubstantiated claims ahead of Mueller's appearances before the House Judiciary and Intelligence Committees.

The news Zebley would join Mueller as his counsel for the House Judiciary hearing and be sworn-in for the House Intel hearing was somewhat last-minute - a fact that seemed to peeve Trump.

The president on Wednesday morning tweeted, "It was NEVER agreed that Robert Mueller could use one of his many Democrat Never Trumper lawyers to sit next to him and help him with his answers. This was specifically NOT agreed to, and I would NEVER have agreed to it. The Greatest Witch Hunt in U.S. history, by far!"

But there's no evidence Zebley is a Democrat. He's registered to vote but has not chosen a party affiliation, according to PolitiFact.

Prior to his work for WilmerHale and the former special counsel's office, Zebley had a successful career at the FBI and as a federal prosecutor.

Read more: GOP Rep. Louie Gohmert screams at Mueller, who offers a 4-word response

After graduating from the University of Virginia's law school, of which Mueller is also an alumnus, Zebley pursued terrorists as part of an FBI unit called I-49 that investigated the American Embassy bombings in Kenya and Tanzania in 1998. Zebley in that capacity helped capture Khalfan Khamis Mohamed, an Al Qaeda member who was convicted in 2001 of taking part in the 1998 bombing of the American Embassy in Tanzania.

Zebley later become a central figure in the FBI's investigation of the September 11, 2001 terror attacks, and in 2006 appeared as the final witness in the trial of Zacarias Moussaoui - who's currently serving a life sentence in prison over his role in helping to plan the attacks. He left the bureau for a short period to serve as an assistant US attorney for the Eastern District of Virginia's National Security and Terrorism Unit.

Mueller in 2011 made Zebley his chief of staff while he was director of the FBI. Zebley also followed Mueller to WilmerHale in 2014 after he left the bureau.

Zebley's wife, Catharine F. Easterly, is a judge on the District of Columbia Court of Appeals.