Washington (CNN) 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.

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