U.S. President Donald Trump ignited a political firestorm on Tuesday by firing FBI Director James Comey, who had been leading an investigation into the Trump 2016 presidential campaign's possible collusion with Russia to influence the election outcome.

The Republican president said he fired Comey, the top U.S. law enforcement official, over his handling of an election-year email scandal involving then-Democratic presidential nominee Hillary Clinton.

The move stunned Washington and raised suspicions among Democrats and others that the White House was trying to blunt the FBI probe involving Russia.

Some Democrats compared Trump's move to the "Saturday Night Massacre" of 1973, in which President Richard Nixon fired an independent special prosecutor investigating the Watergate scandal.

White House officials denied allegations that there was any political motive in the move by Trump, who took office on Jan. 20.

Senate Democratic leader Chuck Schumer said he spoke to Trump and told him he was "making a very big mistake" in firing Comey, adding the president did not "really answer" in response.

An independent investigation into Moscow's role in the election "is now the only way to go to restore the American people’s faith," Schumer said.

Though many Democrats have criticized Comey's handling of the Clinton email probe, they said they were troubled by the timing of Trump's firing of him.

Senator Richard Burr, the Republican chairman of the Senate Intelligence Committee, which is overseeing its own investigation into Russian interference during the election, said in a statement he was also troubled by the timing of Comey's termination.

"His dismissal, I believe, is a loss for the Bureau and the nation," Burr said.

U.S. intelligence agencies concluded in a January report that Russian President Vladimir Putin had ordered an effort to disrupt the 2016 election, with the aim of helping Trump.

CNN reported on Tuesday night that federal prosecutors had issued grand jury subpoenas to former Trump national security adviser Michael Flynn, seeking business records, as part of the probe into Russian interference in the election.

Trump's firing of Comey came a day after former acting Attorney General Sally Yates told a Senate panel that she had informed the White House on Jan. 26 that Flynn was at risk of blackmail by Moscow because he had been untruthful about his discussions with the Russian ambassador, Sergei Kislyak. Trump fired Flynn 18 days later.

Russia has repeatedly denied any meddling in the election and the Trump administration denies allegations of collusion with Russia.