“Aaron Zebley was the Deputy Special Counsel and had day-to-day oversight of the investigations conducted by the Office. He will accompany Special Counsel Mueller to the Wednesday hearings, as was discussed with the committees more than a week ago," according to a statement from a Mueller spokesperson.
He will be there only as counsel to Mueller and will not provide testimony, sources told ABC News.
Witnesses before House committees are generally permitted to have counsel with them.
Initially Tuesday, some Republicans expressed some concern that this was happening at the last minute, but a top Democratic aide said this is nothing new.
"We are exactly where we were a week ago," an aide to a top Democratic committee member told ABC News.
Earlier in the day, Rep. Jim Jordan, R-Ohio, tweeted, "You don't get to change the rules right before kickoff, especially after a 22 month, $30 million investigation."
The Department of Justice declined to comment on Mueller's request.
When asked whether the department had authorized Zebley to appear alongside Mueller, a person familiar said they had not, though it was not immediately clear what steps the DOJ could take to prevent Zebley's appearance, considering his status as a private citizen.