Having hired a host of seasoned federal prosecutors for his probe of the president’s associates, Bob Mueller has now brought on a liaison to Capitol Hill.

Stephen Kelly, formerly the longtime congressional affairs chief for the FBI, has joined the probe, according to several sources familiar with his new role.

“Stephen Kelly is exactly who I would hire if I wanted to share as little information with Congress as possible,” said a Hill staffer who has interacted with Kelly in his new role as Mueller’s congressional liaison.

While Mueller is the face of the investigations into potential coordination between Team Trump and the Kremlin, several Capitol Hill committees are also investigating the issue. And as soon as Mueller’s probe began, concerns emerged that the multiple investigations could butt heads.

So Kelly’s role is key. It’s his job to keep congressional investigators up to date on the special counsel’s probe. He’s also the point of contact for Capitol Hill staffers working on their own, separate Russia-related investigations, and he’s charged with preventing conflict between Mueller’s team and the Hill.

Kelly has been working alongside Mueller since the early days of the special counsel investigation. His role on the probe has not been previously reported.

“We think he’s an asshole,” said a source from a Capitol Hill committee that works closely with the Mueller probe. “It’s unfortunate because the relations with Mueller’s legal team are otherwise strong. The lawyers on each side have worked in the same space for years.”

Kelly undoubtedly has the requisite experience for the role. Before joining the FBI as Assistant Director for its Office of Congressional Affairs, he worked on Sen. Patrick Leahy’s (D-VT) Judiciary Committee staff.

And like other notable members of Mueller’s team––including Andrew Weissmann and Greg Andres––Kelly is an alum of the U.S. Attorney’s Office for the Eastern District of New York, as his FBI bio details.

Kelly’s time at the FBI overlapped with the directorships of both Mueller and James Comey. A former intelligence committee staffer who worked with Kelly said he helped burnish both men’s reputations on the Hill.

“Comey also enjoyed a good relationship with people on the Hill, which you see even now,” the former intelligence committee staffer said, adding that at least some of that friendliness was due to Kelly’s work.

A retired senior FBI official said Kelly briefed Mueller and then Comey  multiple times per week when each led the bureau.

When he represented the FBI on the Hill, Kelly was known as an aggressive, vocal advocate—willing to raise his voice and argue forcefully on behalf of the FBI.

Kelly’s last role at the FBI was in the general counsel’s office, according to the retired official, who said Kelly moved to that position from his post running the Bureau’s congressional relations. An FBI spokesperson declined to comment on Kelly’s work at the Bureau.

Capitol Hill staffers who dealt with Kelly during his time heading Congressional Affairs frequently used the word “tough” to describe him.

“He was tough when he needed to be on behalf of the bureau,” said the former intelligence committee staffer. “Some staffers take that personally, but it’s never personal.”

“If people are asking for things that are unreasonable, he’ll let them know that they’re unreasonable,” the former staffer added.

Another former staffer who worked with Kelly said Hill staffers were often frustrated he wouldn’t tell them more about ongoing FBI investigations, including into the Benghazi attacks and the Boston Marathon bombing.

“I think there will probably be tension with members of Congress who want more detail on where the Mueller probe is going,” that former staffer said.

Other Capitol Hill and administration alums told The Daily Beast this kind of tension is to be expected.

Jamil Jaffer, who as senior counsel for the House intelligence committee worked closely with Kelly and his team, said Kelly’s toughness will be an asset for Mueller.

“Stephen Kelly is a smart, hardworking guy who did a great job representing the FBI on Capitol Hill, and will do the same for Bob Mueller,” said Jaffer, who now heads the National Security Law and Policy Program at George Mason’s Antonin Scalia School of Law.

“The reality is that these positions are tough because they relate to an ongoing, continuing battle between the legislative and executive branches over what information can and should be shared,” Jaffer added. “Stephen Kelly is familiar with that battle and can help congressional staff get the information they need, but he can also tell them when he thinks they don’t need it.”

And Ronald Weich, who was Kelly’s Justice Department counterpart, told The Daily Beast that Kelly is hardworking and savvy.

“Stephen was very good at maintaining a bright-line to protect the integrity of ongoing investigations,” said Weich, who is now dean of the University of Baltimore’s law school. “He felt strongly about it and he was good at it.”