Longtime aide who is joining Mueller during testimony has worked with him for decades

When special counsel Robert Mueller appears before Congress Wednesday, he'll be joined by an aide with whom he's worked for decades.

Aaron Zebley, who was deputy special counsel, is expected to advise Mueller if he needs help with any questions he cannot fully answer himself, CNN has reported.

As Mueller's No. 2, Zebley had "day-to-day oversight" of the investigations conducted by the special counsel's office, according to Mueller's spokesman Jim Popkin. He previously served as Mueller's chief of staff at the FBI and then later following him into private practice.

A graduate of the College of William & Mary in 1992 who earned his law degree in 1996 from the University of Virginia School of Law, Zebley worked as a special agent in the FBI's Counter Terrorism Division before crossing paths with Mueller.

He was part of the FBI team that investigated the September 11th terrorist attacks. He was involved in the trial of al-Qaeda operative Zacarias Moussaoui, who was convicted and sentenced to life in prison for his role in the September 11 attacks. Zebley also played a role in tracking down terrorists behind the twin US embassy bombings in Kenya and Tanzania in 1998.

After seven years as an FBI agent, Zebley went on to become an assistant US attorney in the National Security and Terrorism Unit in the Eastern District of Virginia. He returned to the FBI to serve as special counsel for national security, according to his University of Virginia biography.

Meanwhile, Mueller had been leading the FBI since 2001. When Congress extended his term another two years in 2011, he tapped Zebley as his chief of staff. After Mueller left the FBI, Zebley found himself at the Justice Department as the senior counselor in the National Security Division.

Mueller joined the WilmerHale law firm in 2014, and Zebley joined a week after his former boss. During that time, he assisted Mueller with an inquiry into the NFL's handling of a video of former running back Ray Rice punching his then-fiancée.

Zebley also represented former Hillary Clinton aide Justin Cooper, who helped manage her private server.

When appointed special counsel in 2017, Mueller began assembling a team of lawyers to investigate "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump." He again relied on Zebley, who left WilmerHale to join Mueller's team and begin the investigation that would last nearly two years.

Trump seized on this connection in assailing Zebley's reputation early Wednesday morning ahead of the hearing, claiming that he's a "Never Trumper" without any evidence and pointing to Zebley's involvement representing Cooper.

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