WASHINGTON — Rod J. Rosenstein, the deputy attorney general, told senators on Thursday that he knew President Trump planned to fire James B. Comey as director of the F.B.I. before he wrote a memo outlining the reasons for his dismissal, according to three Democratic lawmakers who were in the briefing.
The meeting with senators came a day after Mr. Rosenstein appointed a special counsel to take over the federal investigation into Russia’s interference in the presidential election and whether any campaign associates of Mr. Trump aided the Russians.
Senators Claire McCaskill of Missouri, Richard J. Durbin of Illinois and Christopher S. Murphy of Connecticut, all Democrats, told reporters immediately after the briefing that Mr. Rosenstein had said Mr. Trump’s decision was made before reviewing a memo that was prepared by the Justice Department.
“There are a lot of missing pieces,” Mr. Durbin said, noting Mr. Rosenstein had left many questions unanswered, specifically regarding what Mr. Trump had said on May 8, and what part Attorney General Jeff Sessions may have played in the decision.
The firing of Mr. Comey last week, and the Mr. Trump’s subsequent acknowledgment that he was thinking about the Russia investigation when he decided to do it, spurred Democrats to step up pressure for a special counsel. The White House had initially said the Russian investigation had nothing to do with Mr. Comey’s dismissal.
Democrats finally got their demand met on Wednesday, following a report in The New York Times detailing a memo that Mr. Comey wrote after he was asked by Mr. Trump in February to drop the investigation into Michael T. Flynn, the former national security adviser who is under scrutiny in the Russia investigation.
Mr. Trump, speaking at the White House after the Senate briefing, said he respected the decision to appoint a special counsel, but the investigation “has been a witch hunt.”
The disclosure by the senators raises the possibility that Mr. Rosenstein could become a witness in the investigation being run by the special counsel, Robert S. Mueller III, a respected former F.B.I. director who served under former Presidents George W. Bush and Barack Obama. The investigation could include an examination of whether Mr. Trump committed obstruction of justice when he asked Mr. Comey to shut down the investigation of Mr. Flynn, and then fired Mr. Comey.
Mr. Rosenstein “faced a stark decision,” Mr. Durbin said. “He could either appoint someone of the stature of Director Mueller, or resign.”