President Trump shakes hands with James B. Comey, the former FBI director, at the White House in January. Trump fired Comey this week. (Andrew Harrer/Bloomberg)

President Trump’s decision to fire James B. Comey as FBI director added new uncertainty to the Senate’s investigation into Russian meddling during the election just as it was beginning to accelerate — but the surprise ouster has also put new pressure on the panel not to slow down.

Comey’s firing “creates challenges” for the Senate Intelligence Committee’s investigation, Chairman Richard Burr (R-N.C.) said this week — particularly because senators want to avoid bumping heads with the FBI over its parallel investigation as both move forward.

At the same time, the sense has grown among committee members that their investigation may be the least compromised by politics and scandal and has become a last line of defense against those forces. The House, which is conducting a third inquiry, is still regrouping from a leadership breakdown earlier this year.

“I think the burden’s always been on our committee because we own the jurisdictional responsibility for the legislative branch to carry it out. And I’ve intended from the beginning, regardless of the hurdles that have been thrown, to finish this investigation,” Burr said the day after Comey’s dismissal.

Sen. Mark R. Warner (D-Va.), the top Democrat on the committee, said Thursday: “The amount of time that we’re both spending and staff’s spending — it is a priority for this committee to get to the bottom of this.”

Outside skepticism remains, however, particularly among Democrats calling for an independent commission, about whether the probe will answer the many questions that have arisen in recent months about Trump’s ties to Russia.

The Senate Intelligence Committee’s probe, which includes an examination of whether Trump associates coordinated with the Russian government, has shown fresh signs of progress in recent weeks. The panel has received at least two caches of information containing raw reports from the intelligence agencies in the last month and requested records from the Treasury Department as well.

In the past few weeks the committee has also added staff to the probe and demanded information from a number of Trump surrogates regarding communications they had with Russians. On Wednesday, panel leaders issued their first subpoena for records from former national security adviser Michael Flynn, who was fired after revelations that he did not disclose communications with Russian officials.

In the wake of Comey’s firing, however, senators anticipate having to renegotiate arrangements with the acting FBI director and his eventual successor, as well as with Deputy Attorney General Rod J. Rosenstein. Rosenstein met Thursday with Burr and Warner, and has agreed to meet with the full Senate next week.

The House Intelligence Committee’s investigation, in contrast, is still regrouping following the decision of Rep. Devin Nunes (R-Calif.), a Trump ally who had been heading the effort, to step down from that role.

Nunes drew widespread criticism for his attempts to focus the House probe away from the possibility of coordination between the Trump campaign and Russia and toward allegations that Trump associates may have been swept up — and identified — in reports of government surveillance of Russians. Democrats, in particular, charged that he had coordinated his efforts with the administration, creating a conflict of interest. After Nunes recused himself, Rep. Michael K. Conaway (R-Tex.) took his place.

Members of the Senate Intelligence Committee are operating with an understanding that to preserve the integrity of their investigation, they must avoid any whiff of partisanship.

“We’re going to do it right because that’s what’s expected of us,” Burr said Thursday.

Michael O’Hanlon, a senior fellow at the left-of-center Brookings Institution, said that for lawmakers investigating Russian meddling, Comey’s firing may trigger a sense of urgency that they are the final backstop, given the uncertainty about the FBI’s future.

Even so, O’Hanlon said he was only about “90 percent confident in the integrity of each” of the three ongoing probes.

The Senate Intelligence Committee has so far mostly resisted pressure coming from various corners of Capitol Hill to move faster, whether coming from Democratic leaders such as Senate Minority Leader Charles E. Schumer (D-N.Y.), or from parallel panels like the Senate Judiciary Committee, which has interviewed many of the principal witnesses who were on the Intelligence Committee’s shortlist as well, down the line.

“What I’m conscious of is how we do the best job in the way that’s the most timely possible,” Warner said last week.

The committee has also rejected calls from many Democrats for an independent commission to take over the congressional probes of the Trump administration. Warner has argued that starting from scratch could undermine the good work the Senate panel has done, though he joined Democrats in endorsing the idea of appointing a special counsel to handle the FBI and Justice Department probes.

Republicans have sought to head off Democrats’ calls for a special prosecutor by arguing that it might impede their work.

“Let us finish our work. It’s moving forward. We’re finally making some significant progress,” said Sen. Marco Rubio (R-Fla.), who is on the committee.

The pace of the Senate Intelligence Committee’s investigation, however, is not theirs alone to determine. How swiftly they are able to pore through source documents from the intelligence agencies depends on how readily and fully they are provided. Scheduling interviews also depends in part on completing those document reviews — and on the cooperation of intended witnesses.

Warner has often noted that the committee’s access to intelligence is “unprecedented,” but he said Thursday that “there’s no playbook” for how to share this much information with a congressional committee.

They are already hitting a few snags.

Comey was supposed to testify before the panel this week as part of an annual briefing on worldwide threats, the timing of which would have allowed committee members to press the director — in public and behind closed doors — on the Russia investigation. Instead, senators grilled his former deputy, acting FBI Director Andrew McCabe, about Comey’s termination. Even the panel’s chairman, Burr, expressed frustrations with how Comey had been treated.

Comey also will not attend a Tuesday closed-door session he had been invited to, Warner told MSNBC on Friday. The former FBI director has not offered senators an alternate date for such a meeting.

“It’s our hope in the not-too-distant future that we can find time for him to come in,” Warner said.

Senators also are facing a delay obtaining information from Flynn, as made clear by the subpoena request Burr and Warner issued for documents this week. Burr has long sought to avoid using subpoenas in this investigation. He noted Thursday that he hopes the one issued for Flynn will be the last, but he also warned that the committee would use all available tools to move the investigation forward.

The committee must also grapple with a mandate that is constantly shifting, as new revelations of the Trump team’s potential ties to Kremlin officials surface in news reports and conflicting narratives emerge from the White House.

Trump’s suggestion that there may be “tapes” of his conversations with Comey posed the latest such potential twist; “if there are tapes,” Warner said on MSNBC. Such tapes could shed new light on the role the FBI’s Russia investigation played in Comey’s firing, but Warner said that he was not convinced that it would be the intelligence panel’s responsibility to investigate them. Instead, he suggested that the Judiciary Committee “ought to get a look at them” as “there may be criminal issues involved here.”

In a letter to White House Counsel Don McGahn, Judiciary Committee ranking member Dianne Feinstein (D-Calif.) wrote Friday that any such recordings, if they exist, “must be preserved by the White House.”

Senior House Democrats also sent a letter Friday to McGahn, in which they “request copies of all recordings in possession of the White House regarding this matter.”

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