WASHINGTON — The F.B.I. director, James B. Comey, took the extraordinary step on Monday of announcing that the agency is investigating whether members of President Trump’s campaign colluded with Russia to influence the 2016 election.

Mr. Comey’s testimony before the House Intelligence Committee created a treacherous political moment for Mr. Trump, who has insisted that “Russia is fake news” that was cooked up by his political opponents to undermine his presidency. Mr. Comey placed a criminal investigation at the doorstep of the White House and said officers would pursue it “no matter how long that takes.”

Joined by Adm. Michael S. Rogers, the director of the National Security Agency, Mr. Comey also dismissed Mr. Trump’s claim that he was wiretapped by his predecessor during the campaign, a sensational accusation that has served as a distraction in the public debate over Russian election interference. Taken together, the two provided the most definitive statement yet that Mr. Trump’s accusation was false.

The New York Times and other news organizations have reported the existence of the investigation into the Trump campaign and its relationship with Russia, but the White House dismissed those reports as politically motivated and rallied political allies to rebut them. Mr. Comey’s testimony on Monday was the first public acknowledgment of the case. The F.B.I. discloses its investigations only in rare circumstances, when officials believe it is in the public interest.