Did President Trump fire FBI Director James Comey for refusing to shut down a criminal investigation against the president’s crony, retired Gen. Michael T. Flynn, who briefly served as national security adviser?

That question is now swirling around the increasingly embattled president after reports indicated that Mr. Comey detailed the attempt to stop the investigation in a memo he wrote to himself following a private meeting with the president.

While President Trump followed a familiar script — deny, deny, deny — as he jetted off on a nine-day trip to the Middle East and Europe, the reports rocked Congress as the administration has been struggling to manage a growing list of scandals.

Veterans in Congress are hearing echoes of Watergate, the scandal that forced President Richard M. Nixon to resign 44 years ago to avoid impeachment.

Some legislators already are labeling President Trump’s action obstruction of justice and calling for his impeachment, though most are treading more carefully.

So far, a House committee has asked the FBI to provide copies of the Comey memo to learn more about what might have happened and whether President Trump’s misstep is as serious as the reports in the New York Times and on NBC News suggested.

Meanwhile, the U.S. Justice Department, led by Trump loyalist Jeff Sessions, took an unexpected step to avoid any appearance that it is under President Trump’s thumb.

The department announced Wednesday that a special prosecutor has been named — former FBI Director Robert Mueller — to take over the probe into allegations that advisers to the 2016 Trump campaign improperly and illegally colluded with Russia to get President Trump elected.

A separate Senate probe is underway in which U.S. Sen. Mark R. Warner of Virginia is playing a major role.

The Comey memo report dropped like a bombshell during a week that began with news that President Trump disclosed classified information to high-ranking Russian visitors to the White House, including that nation’s ambassador.

Mr. Comey has been mum publicly since his firing last week, but friends, upset at his discharge, spread the news about the memo.

According to a friend of Mr. Comey, the former FBI chief wrote the detailed memo on Feb. 14, just a day after President Trump ousted Gen. Flynn for lying to Vice President Mike Pence about his conversations with the Russian ambassador, the same one who met with Mr. Trump.

The New York Times first reported the existence of the memo, which appears to be one of many documents Mr. Comey drafted regarding his conversations with President Trump.

“It’s very rich in detail and hopefully it will come out soon,” the friend of Mr. Comey has told news outlets that agreed to keep his identity confidential.

“There are other memos about his meetings, too. He tried to write down every word President Trump said to him as soon as he could.”

On Capitol Hill, where Republicans have so far generally backed President Trump, a new mood is evident among members. Many expressed deep concern about the latest revelations and demonstrated a new willingness for congressional involvement.

Among the Republicans pulling away from the president is House Oversight Chairman Jason Chaffetz of Utah.

Instead of ignoring the potential scandal, Rep. Chaffetz sent a subpoena to the FBI to produce the memo and also is seeking Mr. Comey’s appearance before the committee.

A Senate committee also wants to hear from Mr. Comey.

The White House pushed back, issuing a tough statement denying the reported allegation.

“While the president has repeatedly expressed his view that Gen. Flynn is a decent man who served and protected our country, the president has never asked Mr. Comey or anyone else to end any investigation, including any investigation involving Gen. Flynn,” the statement read.

“The president has the utmost respect for our law enforcement agencies, and all investigations. This is not a truthful or accurate portrayal of the conversation between the President and Mr. Comey.”

But the accusation against President Trump, whose approval rating has plummeted to historic lows, is one of the most serious since he took office.

The Comey memo was expected by some.

“One thing I learned at DOJ about Mr. Comey: He leaves a protective paper trail whenever he deems something inappropriate happened. Stay tuned,” Matthew Miller, a former DOJ spokesperson, wrote on Twitter after Mr. Comey’s dismissal.

His words proved prescient after President Trump abruptly fired Mr. Comey, despite the FBI’s ongoing investigation into whether Trump’s campaign aides colluded with Russian officials ahead of the election.

The White House gave varying justifications for the firing. The initial explanation was that Pesident Trump fired the FBI director based on a recommendation from top Justice Department officials. But President Trump later refuted this, saying he was going to fire Mr. Comey regardless and with the Russia investigation in mind.

The New York Times earlier reported that President Trump had asked Mr. Comey during a dinner to pledge his loyalty to him, a request that Mr. Comey declined. The White House disputed that account as well.

President Trump then fueled the controversy by suggesting he had his own documentation that could prove damning Mr. Comey, though the president has yet to back up his words.

“James Comey better hope that there are no ‘tapes’ of our conversations before he starts leaking to the press!” President Trump tweeted in response to that story last Friday.

The White House has since refused to comment on whether President Trump is recording conversations in the Oval Office. A number of people close to him admitted they did not know.

Then, adding to the White House’s headaches, news broke on Monday in the Washington Post that President Trump had divulged classified information to Russian foreign minister Sergey Lavrov and Russian Ambassador Sergey Kislyak.

The White House scrambled to contain the fallout, alleging President Trump had done nothing improper even while declining to say whether he had in fact divulged confidential information.

For Democrats on the Hill, the reports on the memo provided further evidence that President Trump’s actions need to be thoroughly investigated.

“Enough is enough. The Congress really needs to get to the bottom of this,” Rep. Adam Schiff of California, the top Democrat on the House intelligence committee, told reporters. “If true, this is yet another disturbing allegation that the president may have engaged in some interference or obstruction of the investigation.”

Senate Minority Whip Dick Durbin, a Democrat from Illinois, called it “stunning, breathtaking to think that a president of the United States would consider reaching out to the head of the Federal Bureau of Investigation and ask them to stop the investigation on anyone, particularly someone who was the national security adviser and is facing allegations that he had undisclosed conversations with the Russians.”

“Each day as this unfolds,” Sen. Durbin continued, “this pattern of obstruction of justice grows.”

Some Republicans acknowledged the growing seriousness of the situation.

GOP Rep. Mike Simpson of Idaho acknowledged that this latest revelation could be serious trouble.

“That would be a problem, if it’s true. That would be a problem,” Sen. Simpson said.

Asked if it is an impeachable offense, he said, “I’m not ready to go there yet. But that would be a problem.”