FBI Director James Comey was seeking to expand his agency's investigation into possible collusion between Russia and U.S. President Donald Trump's campaign in the days before Trump fired him, U.S. media reported on May 10.
The White House has insisted that Comey was fired for making "missteps and mistakes" unrelated to the Russia investigation, which Trump has called "a hoax."
But congressional Democrats on May 10 stepped up their accusations that Comey's removal was intended to undermine the investigation and took steps to try to force Congress to authorize an independent investigation removed from White House influence.
As the two parties battled in Washington, demonstrators gathered in Washington, Chicago, and other cities to demand that the Russia investigation be turned over to independent prosecutors.
U.S. media reported that Comey asked the Justice Department for additional money to hire more people to staff the FBI's Russia probe in the days before he was fired. Comey later informed lawmakers of his request for more funding.
Dick Durbin of Illinois, the Senate's No. 2 Democratic leader, said that he couldn't be certain Comey's funding request triggered his dismissal, but it's clear the FBI "was breathing down the neck of the Trump campaign and their operatives and [the dismissal] was an effort to slow down the investigation."
The Justice Department and White House denied on May 10 that Comey had asked for more funding or that the funding matter had any relation to his firing.
Reuters reported that Comey also infuriated the White House last week by refusing to preview for top Trump aides his planned testimony before a Senate committee.
Reuters, citing White House officials, said Trump, Attorney General Jeff Sessions, and Deputy Attorney General Rod Rosenstein asked Comey for a heads-up about what he would say at a May 3 hearing.
When Comey refused, Trump and his aides considered that an act of insubordination and it was one of the catalysts leading to Trump's decision to fire him, Reuters said.
While the White House denied that Comey's testimony had anything to do with the firing, White House spokesman Sarah Huckabee Sanders on May 10 indicated that Trump viewed Comey as insubordinate, saying Comey had committed "atrocities in circumventing the chain of command."
The administration has said Comey's firing stemmed from his mishandling of an election-year FBI probe into Democratic presidential nominee Hillary Clinton's e-mails. Trump has expressed astonishment that Democrats did not rally behind Comey's dismissal for that reason.
While Democrats have complained about Comey's handling of the Clinton probe -- particularly his decision just days before the November election to announce that he was reopening the probe in a move that Clinton blamed in part for her election loss -- they have nearly unanimously condemned Comey's dismissal because of his critical role this year overseeing the Russia investigation.
Democratic leaders in Congress on May 10 stepped up calls for a special prosecutor to handle the Russia investigation, in light of the White House's removal of Comey.
House Minority Leader Nancy Pelosi said she would try to force a vote on a bill creating an independent committee to investigate the Russia matter if Paul Ryan, the speaker of the House of Representatives, doesn't call up the legislation "immediately" when the House returns from recess next week.
Through a "discharge" petition, a majority of House members can force a House vote on legislation without going through the usual committee procedures, which are controlled by the Republican majority.
Pelosi said the "fireworks at the Department of Justice demand that we remove the investigation from the Trump-appointed Justice Department leadership."
Senate Democratic Leader Chuck Schumer has called on Rosenstein to make good on his pledge to Congress to appoint a special prosecutor if needed to ensure the White House does not try to control the probe.
Senate Republican Leader Mitch McConnell said on May 10 that a special prosecutor isn't needed and appointing one would only interfere with probes already under way in the Senate's Intelligence and Judiciary Committees.
But more than a dozen Republican senators have expressed concern about Comey's firing, including several who said they are open to appointing a special prosecutor.
Some Democrats have compared Trump's move to the "Saturday Night Massacre" of 1973, in which President Richard Nixon ordered the firing of an independent prosecutor investigating the Watergate scandal. The furor that erupted over the firing eventually led to Nixon's resignation.
Democratic Senator Richard Blumenthal told MSNBC that Trump, like Nixon, faces "mounting evidence" against him.
Blumenthal said that evidence increasingly suggests that informal Trump adviser Roger Stone, former Trump campaign manager Paul Manafort, Trump campaign adviser Carter Page, and fired national security adviser Michael Flynn "all face serious criminal culpability, and that evidence if pursued could lead to the very top."
Comey made his first comment since his firing late on May 10, telling FBI staff in a farewell letter that he had "long believed that a president can fire an FBI director for any reason, or for no reason at all," and that he would not spend time dwelling on Trump's decision "or the way it was executed."
Comey is due to testify at a closed hearing of the Senate Intelligence Committee on May 16.
With reporting by AP, Reuters, and New York Times