CAPITOL HILL - U.S. senators of both political parties said Thursday that the public's window into federal probes involving Russia and related matters could be constrained now that a special counsel has been appointed to lead the probe.
Senators spoke after meeting Thursday behind closed doors with Deputy Attorney General Rod Rosenstein, who appointed former FBI Director Robert Mueller to head the Justice Department's investigation of Russian meddling in last year's presidential election and any collusion by the Trump campaign.
WATCH: Graham: Rosenstein to Proceed as Criminal Investigation
"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.
But Senator Richard Durbin, an Illinois Democrat, said Rosenstein told the senators that he knew Trump was going to fire Comey even before he, Rosenstein, wrote the memo that Trump used as justification for the dismissal.
WATCH: Durbin: Rosenstein Knew Comey Would Be Fired Before Writing Memo
Senator Claire McCaskill, a Missouri Democrat, 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, a Texas Republican, who had been considered a contender for FBI director but removed himself from the running, said he thought senators were taking the investigation "enormously seriously." He said he doubted any of his colleagues would try to "delay or impede or impair this investigation in any way."
WATCH: Cornyn: In No One's Interest to 'Delay or Impede' Investigation
Perhaps the most eagerly awaited witness is Comey, who reportedly wrote memos alleging the president pressured him to halt a probe of former national security adviser Michael Flynn. Committees in both houses of Congress have invited Comey to testify publicly.
Need as 'great as ever'
"The need for former Director Comey to come testify in public soon is as great as ever," said the Senate's top Democrat, Chuck Schumer of New York.
But Mueller could nix 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 of Delaware when asked whether Comey would appear before Congress, adding that it remained 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 Democratic Senator Dick Durbin of Illinois. "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."