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The FBI is investigating whether the Trump campaign colluded with a covert Russian operation to interfere with the election, FBI Director James Comey told Congress Monday — and he refuted the president’s baseless claim that his predecessor wiretapped his communications.

Comey’s confirmation in testimony before the House Intelligence Committee that the Trump campaign was the subject of a U.S. counterintelligence investigation was a revelation that could shadow the Trump presidency.

Highlights from Monday's testimony:

For the first time, Director Comey publicly confirmed the FBI is conducting an investigation into whether the Trump campaign “coordinated” with the Russian effort to influence the U.S. election.

Comey debunked the Trump Tower wiretapping allegation.

Comey noted that it is possible for people to collude with a foreign power without knowing it.

Adm. Rogers, the NSA director, dismissed the notion that Britain’s GCHQ intelligence service spied on Trump at Obama’s direction.

The FBI director also dealt the president’s credibility a blow when he said he "has no information that supports" Trump’s allegation from two weeks ago that President Obama ordered surveillance of his communications in Trump Tower during the campaign.

Only courts grant permission for electronic surveillance, Comey told lawmakers, and "no individual in the United States can direct electronic surveillance of anyone."

Terrible! Just found out that Obama had my "wires tapped" in Trump Tower just before the victory. Nothing found. This is McCarthyism! — Donald J. Trump (@realDonaldTrump) March 4, 2017

Comey did not say, however, that no Trump associate was ever picked up by American surveillance. He declined to comment on anything related to surveillance under the Foreign Intelligence Surveillance Act, which allows the FBI to eavesdrop, with a court order, on people they suspect are agents of a foreign power.

NBC News has reported that the FBI has reviewed intercepts showing that Trump aides communicated with Russians during the campaign.

Sitting beside the director of the National Security Agency, Adm. Michael Rogers, Comey began the hearing by revealing in his opening statement that the FBI was in the midst of a counterintelligence investigation into the Russian campaign to hack, leak and promote bogus news stories. Part of that investigation, he said, would examine whether the Trump campaign coordinated with that effort.

Comey said he could not disclose any details about the probe. Normally, he said, the FBI doesn’t confirm or deny investigations, but it can make exceptions in cases of major public interest.

NBC News and other news organizations had reported the FBI’s counterintelligence investigation, but Comey’s public acknowledgement of it opens a dramatic new chapter. Comey did not say how long the investigation might last, but experts say counterintelligence investigations can take years to reach conclusions.

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The Russian campaign was intended to hurt Democrat Hillary Clinton and help Trump, the CIA and other intelligence agencies have concluded. It was also designed to sow chaos and undermine American democracy — something many experts say it has succeeded in doing.

Before the hearing, Trump argued in a series of tweets that "the Russian story" was "made up" by Democrats, and not a topic for serious inquiry.

Comey’s testimony about the wiretapping claim came in response to questioning from the ranking Democrat on the committee, Rep. Adam Schiff of California.

The FBI director carefully but explicitly refuted the claim Trump has continued to embrace since he made it two weeks ago.

On March 4, Trump tweeted, "Terrible! Just found out that Obama had my `wires tapped’ in Trump Tower just before the victory. Nothing found. This is McCarthyism!"

Trump’s efforts to double down on the allegation have forced White House aides trying to defend it publicly and embroiled close American allies in controversy.

Trump and his spokesman, Sean Spicer, made reference to a commentary on Fox News alleging that Obama had asked a British intelligence agency, GCHQ, to eavesdrop on Trump. Over the weekend, the top civilian at the National Security Agency called that claim "errant nonsense."

NSA Director Rogers testified Monday that those claims by Trump and his aides "clearly frustrate a key ally of ours," and he agreed they were not helpful.

Trump made the initial wiretapping charge without evidence — and without consulting top intelligence officials who might have corrected him — in a series of in Saturday morning tweets. He did so, White House officials later told NBC News, after reading a Breitbart news article that was based on a compilation of other media reporting on the U.S. effort to gather information about Russia’s campaign to hack, leak and otherwise interfere in the presidential campaign.

Before Comey spoke, Schiff laid out a detailed rendition of public allegations involving former Trump aides Carter Page and Paul Manafort and their contacts with Russians.

He suggested that July and August of 2016 was a crucial period in the Russian interference campaign, because it marked the time when the Russians began orchestrating a series of leaks designed to embarrass Hillary Clinton and help Donald Trump.

Trust me, it will soon the Podesta's time in the barrel. #CrookedHillary — Roger Stone (@RogerJStoneJr) August 21, 2016

Schiff then repeated a series of assertions from a dossier written by Christopher Steele, a former British intelligence operative who alleged that the Trump campaign engaged in a conspiracy with Russian intelligence officials.

"Here are some of the matters, drawn from public sources alone, since that is all we can discuss in this setting, that concern us and should concern all Americans," Schiff said.

Among other things, Schiff pointed to the tweets of Trump adviser Roger Stone, who boasted that he had communicated with Wikileaks founder Julian Assange. Wikileaks had not yet begun to release emails hacked by the Russian operation from the account of Clinton campaign chairman John Podesta.

In August, Stone tweeted, "Trust me, it will soon be Podesta’s time in the barrel. #Crooked Hillary."

In October, Wikileaks began releasing Podesta’s emails, which proved damaging to the Clinton campaign.

The Democrats made up and pushed the Russian story as an excuse for running a terrible campaign. Big advantage in Electoral College & lost! — Donald J. Trump (@realDonaldTrump) March 20, 2017

Later, he concluded, "Is it possible that all of these events and reports are completely unrelated, and nothing more than an entirely unhappy coincidence? Yes, it is possible. But it is also possible, maybe more than possible, that they are not coincidental, not disconnected and not unrelated, and that the Russians used the same techniques to corrupt U.S. persons that they have employed in Europe and elsewhere. We simply don’t know, not yet, and we owe it to the country to find out."

White House spokesman Sean Spicer called Schiff’s opening statement "a series of mistruths and misdirection."

Before the hearing, Trump argued in a series of tweets that "the Russian story" was "made up" by Democrats, and not a topic for serious inquiry. Comey’s disclosure of an ongoing investigation demolishes that argument.

Before Comey spoke, the ranking Democrat on the committee, Rep. Adam Schiff, laid out a detailed rendition of public allegations involving former Trump aides Carter Page and Paul Manafort and their contacts with Russians.

On Monday afternoon, after Comey's testimony, Manafort issued a statement denying any role in any cyberattack on the Democratic National Committee or leaks of DNC emails, and denying that he ever spoke to any Russian official or anyone else claiming involvement in the attack. "The suggestion that I ever worked in concert with anyone to release hacked emails or sought to undermine the interests of the United States is false," said the statement. "Despite the constant scrutiny and innuendo, there are no facts or evidence supporting these allegations, nor will there be."