U.S. Deputy Attorney General Rod Rosenstein returns to Capitol Hill Friday for a closed door briefing with members of the House of Representatives about his appointment of former FBI Director Robert Mueller to lead the independent investigation into possible collusion between Donald Trump’s presidential campaign and the Russians.

Rosenstein’s appearance before the House comes one day after privately briefing the entire Senate.

After Thursday’s meeting, senators of both political parties said the public’s window into multiple ongoing congressional investigations into Russia’s meddling in last year’s presidential election and any collusion by the Trump campaign could be constrained now that a special counsel has been appointed to lead the Justice Department’s probe into the matter.

“Congress’ ability to conduct investigations of all things Russia has been severely limited,” said Republican Senator Lindsey Graham of South Carolina. “If I were Mr. Mueller, I would jealously guard the witness pool. So, one of the biggest losers in this decision is the public.”

Rosenstein's appearance in the Senate was originally meant to allow him to explain why Trump abruptly fired FBI Director James Comey last week, but the focus changed when Rosenstein appointed Mueller as special counsel Wednesday night.

FILE - FBI Director James Comey makes a statement FILE - FBI Director James Comey makes a statement at FBI Headquarters in Washington, Tuesday, July 5, 2016. FILE - FBI Director James Comey makes a statement at FBI Headquarters in Washington, Tuesday, July 5, 2016.

But Senator Richard Durbin said Rosenstein told the senators that he knew Trump was going to fire FBI Director Comey even before Rosenstein wrote the memo that Trump used as justification for the dismissal.

Senator Claire McCaskill told reporters Rosenstein was "very careful about not going into any details surrounding the removal of Comey because he wants to give Robert Mueller the opportunity to make an independent decision" about how to move forward on the case.

And Senator John Cornyn, who had been considered a contender for FBI director but removed himself from the running, said he believes senators are taking the investigation "enormously seriously." He expressed doubt any of his colleagues would try to "delay or impede or impair this investigation in any way."

Perhaps the most anticipated witness is former FBI Director Comey, who was fired by Trump and reportedly wrote memos alleging the president pressured him to halt a probe of Michael Flynn, Trump's former national security adviser. Committees in both houses of Congress have invited Comey to testify publicly.

“The need for former director Comey to come testify in public soon is as great as ever,” said Chuck Schumer, the Senate’s top Democrat.

Then FBI Director Robert Mueller testifies on Capi Then FBI Director Robert Mueller testifies on Capitol Hill in Washington, June 19, 2013, before the Senate Judiciary Committee. Then FBI Director Robert Mueller testifies on Capitol Hill in Washington, June 19, 2013, before the Senate Judiciary Committee.

But Mueller could put off such an appearance, wanting to keep Comey’s testimony out of the public eye until the special counsel’s work is done.

“I think that’s an open question,” said Democratic Senator Chris Coons, when asked if Comey would appear before Congress, adding that it remains to be seen “how ongoing congressional investigations will be coordinated with the special counsel.”

“Mueller’s in charge, completely in charge of this investigation,” said Durbin. “But beyond that, we’re going to have to work this thing out.

“I don’t know where this will go. But I trust Mueller,” Durbin added.