FBI Director James Comey stated unequivocally on Monday that he has 'no information' to back up President Donald Trump's claim that President Obama wiretapped Trump Tower.

He made the statement at a dramatic hearing of the House Intelligence Committee - and also told it that there had been an active counter-intelligence investigation into claims of collusion between the Trump cmapaign and Russian intelligence since last July.

Flanked by Admiral Mike Rogers, director of the National Security Agency he gave the damning verdict on the tweets which have dominated Washington for two weeks shortly after the beginning of the hearing.

The twin disclosures from the FBI director lit a fire under the White House - prompting a furious defense of the original tweet, and for the president to use his Twitter account to highlight the parts of Comey's evidence which were in his favor.

Asked if the president would now apologize for his tweet after Comey, who spoke for the Department of Justice as well as the FBI, and the director of the NSA, discredited it, Press Secretary Sean Spicer said there would be no apology.

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FBI Director James Comey took the unusual step of confirming the existence of a counterintelligence investigation today at a House hearing on Russia meddling in the 2016 election

Trump's @POTUS account meanwhile went after the Obama administration for the leaking of Michael Flynn's name and pre-inauguration conversations with the ambassador from Russia.

Pushing on: President Trump, who welcomed Iraqi prime minister Haider al-Abadi to the Oval Office, will not apologize for his original incendiary tweet

'We started a hearing. It's still ongoing,' Sean Spicer said nearly four hours into the Comey grilling.

'I think there's a lot of areas that still need to be covered. There's a lot of information that still needs to be discussed.'

The White House official said the House is still at the beginning of its investigative process and there will be more hearings. He predicted once again that the president would be vindicated in the end.

Trump's @POTUS account meanwhile went after the Obama administration for the leaking of Michael Flynn's name and pre-inauguration conversations with the ambassador from Russia, pointing the finger again at the former president.

With respect to the president's tweets about alleged wiretapping directed at him by the prior administration, I have no information that supports those tweets James Comey, FBI Director

'FBI Director Comey refuses to deny he briefed President Obama on calls made by Michael Flynn to Russia,' one of the POTUS tweets said.

The statement, repeated later by Spicer, was an attempt to link the Obama White House to illegal leaking that officials acknowledged at today's hearing hurt national security.

Flynn's links to Russia emerged just before the inauguration, and led to his resignation for lying to Vice-President Mike Pence.

Rep. Adam Schiff (D-Calif.), the top Democrat on the House Intelligence Committee, had questioned Comey at the start of the blockbuster public hearing, asking whether Trump's tweet alleging that he had his phone tapped was a 'true statement.'

Schiff read the entirety of the quote from Trump's tweet, including Trump's claim that 'this is McCarthyism.'

'Director Comey, was the president's statement that Obama had his wires tapped in Trump Tower a true statement?' Schiff asked the FBI director.

'With respect to the president's tweets about alleged wiretapping directed at him by the prior administration, I have no information that supports those tweets,' Comey said.

Comey went further, stating that the Justice Department had asked him to share that the answer is the same for the DoJ and 'all its components,' he said.

The Department also 'has no information that supports those tweets'.

Comey also took the unusual step of confirming the existence of a counterintelligence investigation at the House hearing on Russia meddling in the 2016 election.

He said the FBI is looking at possible links and coordination between President Donald Trump's 2016 campaign and the Russians. The investigation includes an assessment of potential crimes that were committed.

Because the investigation is ongoing, and it deals with classified information, Comey said he could not share additional details about who the target of the probe is and what conduct is under scrutiny in a public setting.

'I know that is extremely frustrating to some folks, but it is the way it has to be,' he said.

He later said that it had been launched in July but lawmakers had not been informed of it because of its 'sensitivity'.

And yet, Spicer told reporters on Monday afternoon, 'It's clear that nothing has changed.'

Senior Obama intelligence officials have gone on record to confirm that there is no evidence of a Trump Russia collusion. The Obama CIA Director said so, Obama's Director of National Intelligence said so, and we take them at their word Sean Spicer, White House Press Secretary

'Senior Obama intelligence officials have gone on record to confirm that there is no evidence of a Trump Russia collusion. The Obama CIA Director said so, Obama's Director of National Intelligence said so, and we take them at their word.'

In response to Comey's assertion that aspects of the Trump campaign apparatus were being probed, Spicer said the media was making assumptions about that might mean.

No White House officials are being investigated to his knowledge, Spicer said. He put as much distance as possible between individuals who are already known to be under scrutiny - although not FBI investigation - and the president as possible, calling one of them, Carter Page, a 'hanger on.'

The other two are Roger Stone and Paul Manafort, and Spicer launched a concerted operation to distance them both from the White House.

Roger Stone remains a friend of the president's, but he hasn't been affiliated with the campaign since 2015, Spicer said.

Former campaign manager Paul Manafort was with Trump for eight weeks and did not play a 'lasting role' in the president's victory, an even clearer snub.

'So my point is to suggest now, that if you look at the final three months of the campaign, where none of the individuals in question that Democrats brought up over and over today, were affiliated with the campaign, to suggest that somehow shows some high level collusion is a bit of the stretch to say the least,' Spicer said.

Republicans on the committee are using the hearing to try to establish the source of leaks to news publications about the communications between Trump associates and Russian officials, like Flynn.

Comey and National Security Agency Director Mike Rogers (right) arrive to speak during the House Permanent Select Committee on Intelligence hearing on Russian actions during the 2016 election campaign

Legislators also pushed Comey to explain the intelligence community's assessment that Russian hacking was meant to hurt Hillary Clinton and help Trump win the election.

He said that one only had to use 'logic' to arrive at the conclusion that Russian President Vladimir Putin did not want the former secretary of state to win.

KEY MOMENTS FROM THE HEARING ON PRESIDENT TRUMP'S CLAIM THAT FORMER PRESIDENT OBAMA WIRETAPPED HIM: 'With respect to the president's tweets about alleged wiretapping directed at him by the prior administration, I have no information that supports those tweets, and we have looked carefully inside the FBI. The Department of Justice has asked me to share with you that the answer is the same for the Department of Justice and all its components.' - Comey 'Let me be clear: we know there was not a wiretap on Trump Tower. However, it´s still possible that other surveillance activities were used against President Trump and his associates.' - Committee Chairman Representative Devin Nunes ON THE FBI INVESTIGATION OF RUSSIA'S INTERFERENCE: 'I have been authorized by the Department of Justice to confirm that the FBI, as part of our counterintelligence mission, is investigating the Russian government's efforts to interfere in the 2016 presidential election and that includes 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.' - Comey ON UNVERIFIED CLAIMS THAT OBAMA ASKED BRITAIN'S GCHQ TO EAVESDROP ON TRUMP: 'That would be expressly against the construct of the Five Eyes (intelligence-sharing) agreement that's been in place for decades. ... I have seen nothing on the NSA side that we engaged in any such activity.' The claim 'clearly frustrates a key ally of ours.' - Rogers ON LEAKS OF CLASSIFIED INFORMATION: 'Numerous current and former officials have leaked purportedly classified information in connection to these questions. We aim to determine who has leaked or facilitated leaks of classified information so that these individuals can be brought to justice.' - Nunes ON THE U.S. INTELLIGENCE JUDGMENT THAT RUSSIA FAVORED TRUMP OVER DEMOCRATIC OPPONENT HILLARY CLINTON: 'I think that was a fairly easy judgment for the community. Putin hated Secretary Clinton so much that the flip side of that coin was he had a clear preference for the person running against the person he hated so much.' - Comey ON WHETHER RUSSIAN INFLUENCE DETERMINED THE ELECTION OUTCOME: 'We will never know whether the Russian intervention was determinative in such a close election. ... What does matter is this: The Russians successfully meddled in our democracy and our intelligence agencies have concluded they will do so again.' - Ranking committee Democrat Representative Adam Schiff Advertisement

'That was a fairly easy judgement for the community, Putin hated Secretary Clinton so much, that that the flip side of that coin was he had a clear preference for the person running against the person he hated so much,' Comey stated.

Also at issue in the hearing was whether the FBI has any evidence to support Trump's claims that Obama spied on him - Comey says he didn't.

Pressed on whether he personally engaged in any type of 'McCarthyism,' Comey responded with a joke.

'I try very hard not to engage in any -isms of any kind, including McCarthyism,' he said.

'I'm not going to try and characterize the tweets themselves; all I can tell you is that we have no information that supports him,' Comey said.

Schiff also tried to get Comey to comment on Trump's question about whether it was 'legal' for a sitting president to be 'wiretapping a race for president prior to an election. Trump had written that it was: 'turned down by a court earlier' and called it 'a new low.'

Then he went about trying to establish whether it was even possible for President Obama to order such a wiretap.

Asked whether it would be legal for Obama to have authorized such a tap, Comey responded: 'I'm not going to characterize or respond to the tweets themselves. I can tell you in general, as [NSA director] Admiral Rogers and I were just saying, there is a statutory framework in the United States under which courts grants permission for electronic surveillance, either in a criminal case or in a national security case, based on a showing of probable cause.'

He continued: 'Carefully overseen, it's a rigorous, rigorous process that involves all three branches of government. And it's one we've lived with since the late 1970s. That's how it works.'

'No individual in the United States can direct surveillance of anyone. It has to go through a application process. Ask a judge. The judge can then make the order.'

Schiff followed up: 'So Obama could not unilaterally order a wiretap of anyone?'

'No president could,' Comey responded.

Then, Schiff tried to push Comey on Trump's claims that Obama's unsubstantiated activity approximated Watergate, asking about the 'graveman' of the situation.

'The gravamen of it was an abuse of power including break-ins, unlawful wiretaps, obstruction of justice. Sort of the cycle of criminal conduct,' Comey said, noting that he had studied Watergate.

Then Schiff – who in his introductory remarks called Trump's tweets a 'slanderous accusation' – tried to make his own Watergate analogy.

'But there is evidence is there not, of a break-in of the Democratic headquarters by a foreign power, using cyber means?' he asked the FBI director.

'Yes there was,' Comey responded.

Comey was tight-lipped when Schiff and other lawmakers tried to get him to comment on the investigation of Trump campaign alleged Russia ties.

Asked about informal Trump advisor Roger Stone's prediction that it would soon be Clinton campaign chair John Podesta's 'time in the barrell' in advance of the release of his hacked emails, Comey said: 'I believe that's correct chronology.'

How Stone could have known Podesta's emails would be released is something Comey said he could not comment on.

Trump said in one of the messages that Obama 'was tapping my phone'

The FBI director got questioned about Trump informal advisor Roger Stone's prediction that Hillary Clinton campaign chair John Podesta's 'time in the barrel' was coming in advance of the leak of his hacked emails. 'I believe that's correct chronology,' Comey responded

Ranking member Rep. Adam Schiff (D-CA) speaks during the House Permanent Select Committee on Intelligence questioned FBI Director Comey about unproven accusations of wiretapping and alleged Trump camp Russia ties

'I try very hard not to engage in any isms of any kind, including McCarthyism,' FBI Director James Comey said when asked about President Trump's charge that President Obama engaged in 'McCarthyism'

Stone has denied knowing the emails were coming out, although he admitted to Twitter communications with Guccifer 2.0, who published hacked emails and is believed to be a front for Russian intelligence.

Comey would not comment on anything regarding potential collusion in the election, other than to say it was being investigated.

The law enforcement officer preemptively asked lawmakers not to draw any conclusions about his inability to share details or make comparisons to his willingness to talk about already concluded investigation - a subtle acknowledgement of his testimony before Congress after the FBI finished its probe into Clinton.

Comey and Admiral Mike Rogers, head of the NSA, also warned leakers within the intelligence community that their conduct is illegal and will be met with serious consequences if they are caught.

'If I become aware of any such conduct, there is no place for you on this team. It is unacceptable to citizens of the nation,' Rogers said.

Comey said leaking is a 'serious, serious crime' and he too has stressed that 'it simply cannot be tolerated.'

Trump's White House had asked the panel to investigate both the leaking and the wiretapping allegations as part of its review of Russian interference in the election.

A Feb. 14 New York Times article alleged that Trump campaign operatives were in constant contact with the Russians - prompting an outcry from the Trump White House. Not only was the information in the article false, it said, the charges were based on government information that should not have been shared with journalists.

Two days later, the Washington Post reported that Flynn was dishonest about his talks with the Russian ambassador.

Flynn told the FBI, and before that, the vice president, that he did not discuss sanctions with Sergey Kislyak. Routine surveillance of Kislyak called Flynn's account into dispute. The Post first reported on those conversations on Feb. 9.

Flynn was subsequently dismissed, not for the contact, but for his deceit, Trump said.

Comey and Rogers revealed Monday that dozens of intelligence operatives would have had access to the classified information.

Rogers said 19 individuals, other than himself, within the NSA could have done the leaking.

The attorney general, director of national intelligence, FBI director, and other Justice Department employees have authorization, as well.

Comey was unable to define the scope of access within the FBI but acknowledged that former AG Loretta Lynch and Sally Yates, the acting AG Trump fired early in his tenure, were among those who would have access to technology that unmasks the names of individuals in conversation with foreign operatives.

The White House sicked on that claim, with Spicer later telling reporters that 'certain political appointees in the Obama administration had access to the names of unmasked U.S. citizens' like Flynn.

'Not only was General Flynn's identity made available. Director Comey refused to answer the question on whether or not he'd actually briefed President Obama on his phone calls and activities,' Spicer said.

Aside from that acknowledgement - that any number of people could be behind the leaking - testimony fell broadly along the lines that lawmakers expected, and politicians used their time to highlight aspects of the investigation that cast doubt on claims made by the other side.

House Intelligence chief Devin Nunes gave swift opening remarks before passing the baton on to Schiff, who delivered nearly 15 minutes of remarks reviewing the allegations against Trump associates and the Russian government.

Schiff, the top Democrat on the panel, had accused Trump of leading Congress on a 'wild goose chase' in a Sunday interview on NBC's Meet the Press.

The Justice Department complied on Friday with a demand from the committee to turn over evidence relating to Trump's claims that Barack Obama wiretapped him. It sent copies of the materials to a companion committee in the Senate, as well.

Schiff says he received a classified briefing on the documents Friday after most lawmakers had left town.

'Once again, no evidence to support the President's claim that he was wiretapped by his predecessor,' he said.

Making reference to a colleague's claim that legislative bodies need to 'get to the bottom' of Trump's allegations, Schiff said: 'We are at the bottom of this.'

'There is nothing at the bottom,' he added.

On Sunday, and again Monday, Nunes also acknowledged that the FBI provided no evidence on Friday of a warrant to wiretap Trump Tower.

The only electronic surveillance his committee knows of, Nunes said, was the monitoring of calls between Flynn and the Russian ambassador.

'There was no FISA warrant that I'm aware of to tap Trump Tower,' the California Republican stated on Fox News Sunday.'

TIME TO APOLOGIZE? Republican Rep. Will Hurd says Trump needs to tell Barack Obama he's sorry for claiming that he spied on him

He also said: 'Was there a physical wiretap of Trump Tower? No, there never was. The information we received Friday continues to lead us in that direction.'

'If you take the president literally, it didn't happen.'

Will Hurd, a Republican on the panel, said in a Sunday morning appearance that he believes it's time for Trump to apologize for the incredible assault on the former president.

'It never hurts to say you're sorry,' Hurd, a former undercover CIA operative, told ABC's George Stephanopoulos. 'It's not just sorry to the president, but also to the U.K. for the claims or the intimation that the U.K. was involved in this, as well.'

Trump's White House on Thursday brought British intelligence into the saga when it referred to a report on Fox & Friends that claimed Obama had the GCHQ spy on Trump for him.

The British government immediately slapped down the claim, made by Fox's Judge Andrew Napolitano.

White House Press Secretary Sean Spicer nevertheless repeated it as he read off a list of news reports on Thursday that purportedly supported the president's claim that Obama spied on him.

Britain's spy service shredded the charge.

A GCHQ spokesperson said the claims were 'nonsense.'

'They are utterly ridiculous and should be ignored,' its press officer said in a statement.

Admiral Rogers backed that today, asked directly if he agreed with GCHQ's statement, he replied that he did.