WASHINGTON — Aaron Zebley, a top prosecutor in the special counsel’s office, is so close with his former boss, Robert S. Mueller III, that he wears the same kinds of pinstripe suits, white dress shirts and Casio watch.
On Wednesday, Mr. Zebley sat at the witness table with Mr. Mueller during two highly anticipated hearings on the Russia investigation, an unusual arrangement that House Democrats signed off on Tuesday after Mr. Mueller made a last-minute request. Mr. Zebley, 49, was to help Mr. Mueller answer questions about the sprawling investigation and the 448-page report documenting it, a committee official familiar with the hearing said.
[Mr. Mueller testifies before Congress.]
The news prompted President Trump to post a series of accusations about Mr. Zebley, including that he is a “Never Trumper” lawyer, a reference to prosecutors on Mr. Mueller’s team he believes were opposed to him. There is no evidence of that claim. Mr. Zebley is not politically affiliated and according to a Washington Post review, has not donated to candidates from either party.
Republicans on the House Judiciary Committee also criticized the request as a circumvention of House rules. Representative Doug Collins of Georgia, the top Republican on the panel, said Democrats were “outsourcing” Mr. Mueller’s responsibility to testify.
Mr. Zebley was Mr. Mueller’s chief of staff when he was F.B.I. director and left the bureau with him in 2013. The two joined the law firm WilmerHale the following year, keeping offices next to each other and conducting investigations for the same corporate clients. Then Mr. Zebley again played a chief of staff-like role at the special counsel’s office.
Mr. Zebley was chasing Al Qaeda well before the Sept. 11 attacks.
Mr. Zebley, who like Mr. Mueller graduated from the University of Virginia’s law school, was part of a special unit of F.B.I. agents called I-49, which investigated the American Embassy bombings in Kenya and Tanzania in 1998. He helped capture one of the nation’s most wanted terrorism suspects, Khalfan Khamis Mohamed, a member of Al Qaeda who was charged in 1999 for his role in the bombing of the American Embassy in Tanzania.
Mr. Zebley’s work became central to the F.B.I.’s reorganization after the Sept. 11, 2001, attacks. Before climbing the ranks of Mr. Mueller’s small circle of senior aides, he became a federal prosecutor in the National Security and Terrorism Unit in Alexandria, Va.
Prosecutors turned to him in 2006 as the final witness in the trial of Zacarias Moussaoui, who was accused of helping plot the Sept. 11 attacks. Mr. Zebley, who was one of the lead agents in the investigation of Mr. Moussaoui, testified that information that investigators had discovered after the attacks could have provided important leads beforehand.
He has long been Mr. Mueller’s right hand.
Mr. Zebley, whose wife, Catharine F. Easterly, is a judge on the District of Columbia Court of Appeals, was chief of staff to Mr. Mueller in the final years of his tenure at the F.B.I. The two were virtually inseparable in meetings and many congressional hearings, where Mr. Zebley sat dutifully behind his boss.
When the two rejoined forces at WilmerHale in 2014, they worked with large corporate clients on internal investigations, including the National Football League. Mr. Zebley helped the N.F.L. assess whether it had responded appropriately to domestic violence allegations against Ray Rice, the former star running back for the Baltimore Ravens.
Mr. Mueller also employed Mr. Zebley as a speechwriter, a job for which Mr. Zebley reported $10,000 in 2016 income on a financial disclosure.
And Mr. Zebley played a key role in the Russia investigation.
Mr. Zebley left his $1.4 million-a-year job at WilmerHale as one of the first hires in the Russia investigation. Two other prosecutors on Mr. Mueller’s team, James L. Quarles III and Jeannie Rhee, also left the firm at the same time.
As deputy special counsel, Mr. Zebley was less a prosecutor than a kind of steward of the investigation. He had “day-to-day oversight of the investigations conducted by the office,” said Jim Popkin, a spokesman for Mr. Mueller. On the calendars of one of the team’s prosecutors, Mr. Zebley’s initials appear 111 times and next to many “team leader” meetings, suggesting he may have led them.
When Ed O’Callaghan, a senior official at the Justice Department who monitored the Russia investigation, visited the special counsel’s offices for briefings, he met with Mr. Zebley, who also accompanied Mr. Mueller to meetings on Capitol Hill and at the Justice Department.
Mr. Zebley made a rare appearance in court in January 2018, when the special counsel’s office sought to keep secret the memos written by the former F.B.I. director James B. Comey.