Deputy Attorney General Rod Rosenstein (center) arrives for a private meeting with the Senate at the U.S. Capitol on Thursday. (T.J. Kirkpatrick/Bloomberg)

Deputy Attorney General Rod J. Rosenstein’s meeting with senators on Thursday grew especially heated when he faced questions from two frustrated liberal Democrats, according to multiple people familiar with the meeting.

Rosenstein came to Capitol Hill to discuss the firing of FBI Director James B. Comey and his decision to hire Robert Mueller as a special counsel to probe Russian meddling in last year’s elections. He briefed U.S. senators in what they call “the SCIF,” or a Sensitive Compartmented Information Facility, a below-ground facility in the Capitol Visitors Center that has an area large enough to hold all 100 senators.

In that space, Rosenstein faced especially aggressive questions from Democrats about the scope of the investigation set to be led by Mueller; why Attorney General Jeff Sessions was involved in the firing of Comey; and why Sessions is helping select the new director, according to multiple people.

Sens. Kirsten Gillibrand (D-N.Y.) and Al Franken (D-Minn.) especially grilled Rosenstein on Sessions’s role and the scope of Mueller’s new investigation, according to three senators in the room who spoke about the meeting on the condition of anonymity. Another aide familiar with the exchange described Franken as “heated.” But one of the senators asked to recount the meeting said that Franken and Gillibrand were “passionate” — not rude.

“They weren’t inappropriate. They were asking strong questions that were strongly felt. I didn’t think anybody was inappropriate,” the senator said.

Franken’s spokesman confirmed he “was passionate about getting to the bottom of what happened.” A Gillibrand spokesman couldn’t be reached for comment.

Rosenstein confirmed that Mueller’s investigation will have a broad scope — encouraging news to Democrats especially eager for an investigation to cover all potential angles. Because of the wide berth, Rosenstein referred several of the senators’ questions to Mueller, which caused much of the frustration in the room.

Rosenstein “was very careful about not going into any details surrounding the removal because he wants to give Robert Mueller the opportunity to make an independent decision” about how to proceed, said Sen. Claire McCaskill (D-Mo.), who told reporters she thought that was an appropriate move.

Sen. Ron Johnson (R-Wis.) said that Rosenstein refused to answer “completely appropriate” questions asked by his colleagues, a sign that it may become more difficult for the Senate Intelligence Committee’s investigation to continue.

“Sometimes you’ve got to be careful for what you ask for, because when you get it, then you can’t get answers to things because it’s now an active investigation, there’s a special prosecutor in place and I think we’ll have a more difficult time getting information,” Johnson said.

Sen. Bill Cassidy (R-La.) emerged to tell reporters that some colleagues asked “provocative” questions — but he refused to detail the exchanges. He said that Rosenstein had “hesitated to characterize” Mueller’s investigation “because it could be perceived that somebody committed a crime. He stressed that’s not the case — that it’s an investigation.”

Other senators said that Mueller’s investigation is considered a criminal probe given that the laws dictating the appointment of a special counsel say that they are supposed to investigate criminal matters.

Several Republican senators asked Rosenstein if the Intelligence Committee could continue its work now that Mueller is conducting his investigation. Rosenstein was “unequivocal” that the panel can and should continue its investigation, according to the senators and staffers familiar with the meeting.

Rosenstein will hold a similar briefing for House lawmakers on Friday.