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.
“This is one of those circumstances,” Mr. Comey said.
Mr. Comey said the F.B.I. was “investigating the nature of any links between individuals associated with the Trump campaign and the Russian government, and whether there was any coordination between the campaign and Russia’s efforts.”
Counterintelligence investigations are among the F.B.I.’s most difficult and time-consuming cases, meaning an investigation could hang over the Trump administration for years even though such inquiries rarely lead to criminal charges.
American intelligence agencies concluded in January that President Vladimir V. Putin of Russia personally ordered a covert effort to hurt Hillary Clinton’s chances and aid Mr. Trump. That included hacking political targets, including the Democratic National Committee, and releasing embarrassing emails through the website WikiLeaks.
The White House dismissed most of Mr. Comey’s testimony, saying there was no coordination between the Trump campaign and Russia and so there was nothing to investigate. Sean Spicer, the White House press secretary, said the more pressing issue was who disclosed classified information about Mr. Trump’s advisers to journalists, suggesting that they might have been former members of the Obama administration.
American officials have said that they have so far found no proof of collusion between the Trump campaign and Russia, but current and former officials say they have uncovered evidence that Mr. Trump’s associates were in repeated contact with Russian officials — including people linked to Russian intelligence.
Roger J. Stone Jr., a longtime adviser to Mr. Trump, has acknowledged communicating with Guccifer 2.0, an online persona believed to be a front for Russian intelligence officials involved in disseminating hacked Democratic emails. Mr. Stone has denied that there was anything improper about the contact, and he was one of many, including political operatives and journalists, to communicate with the hackers.
Last July, the month that WikiLeaks began releasing the hacked emails, Carter Page, a foreign policy adviser to Mr. Trump, visited Moscow for a speaking engagement. Mr. Page has declined to say whom he met there, but he has said they were mostly scholars.
Michael T. Flynn, a Trump campaign adviser who went on to be his national security adviser, was paid more than $65,000 by companies linked to Russia in 2015, including an American branch of a cybersecurity firm believed to have connections to Russia’s intelligence services, according to congressional investigators. Mr. Flynn was forced to resign after misrepresenting his conversations with the Russian ambassador to the United States.