Comey delivered his assessment without a blink, telling lawmakers neither the FBI nor the Justice Department had evidence to support such allegations.

Watch: FBI director James Comey confirms Russia investigation

Intensely concentrated, with furrowed brow, the towering Comey – he stands two metres tall – took the heat during a marathon first public hearing on the issue of Russian meddling in last year’s election, and Trump’s unsubstantiated allegations of wiretapping.

But the FBI chief, who has been in his post since September 2013, is also a highly skilled political operator, who knows his words carry weight.

The Democrat Clinton learned that the hard way, when Comey called a surprise press conference last July to deliver a dressing-down over her use of a private email server that reverberated all the way to the November polls.

When Trump decided to keep the Obama appointee in his job, it raised eyebrows from critics who saw it as a tacit reward for the part he played in damaging Clinton’s chances. But the FBI chief increasingly looks to be a thorn in the president’s side.

Comey has now set his sights on the issue of Russian election meddling, which has stalked Trump’s young presidency. And if there is one character trait the FBI chief is known for, it is tenacity.

Under Obama, Comey repeatedly stole the spotlight from his boss, former attorney general Loretta Lynch, who was reduced in the Clinton case to announcing she was following his advice not to press charges.

In the wake of the 2014 fatal police shooting of unarmed black teen Michael Brown in Ferguson, Missouri, Comey raised hackles by supporting cops who were wary of fulfilling their duties, for fear of their actions being caught on video.

Many top US government careers begin in New York, and Comey is no exception – he hails from the Manhattan suburbs. He cut his teeth as a federal prosecutor in New York and the Washington area.

In 2003, the father of five became deputy attorney general. The following year, he faced one of his toughest showdowns, confirming his reputation for being independent and unafraid.

Comey had become acting attorney general due to the illness of his boss John Ashcroft.

At Ashcroft’s bedside, the presidential counsel to George W. Bush, Alberto Gonzales, was trying to persuade him to reauthorize a controversial warrantless eavesdropping programme.

Comey – who was against extending the programme – later revealed the incident to senators, unleashing a political firestorm.

Trump twists FBI chief’s words on Russia

US President Donald Trump produced a running commentary on Monday on FBI director James Comey’s testimony to Congress. Thanks to the length of the hearing and the immediacy of Twitter, Comey was able to comment on the president’s commentary without leaving his seat. It was a nearly real-time exchange that circled back on itself, like a cat chasing its tail.

Trump tweet: “The NSA and FBI tell Congress that Russia did not influence electoral process.”

Fact check: No such assurance was offered by Comey or his fellow witness at the hearing, NSA director Mike Rogers. They did not offer any conclusions about whether Russia succeeded in influencing the election.

Comey: Asked about the tweet while he was still testifying, he voiced “no opinion” on Russia’s influence. “We have no information on that subject,” he said.

“It’s not something we looked at.”

He confirmed, though, that the FBI is investigating whether and how Russia tried to shape the election, a probe he says includes any potential coordination between Trump campaign associates and Russia’s government.

Trump tweet: “FBI director Comey refuses to deny he briefed President Obama on calls made by Michael Flynn to Russia.”

Fact check: “Refuses to deny” is true, but perhaps misleading. Throughout the hearing, Comey declined to deny any assertion made by his questioners that mentioned Flynn or any other individuals, explaining that the FBI is not in the business of correcting or verifying such reports.

Flynn helped Trump in the campaign, became national security adviser at the start of Trump’s presidency and was fired after he was found to have misled senior members of the administration about his contacts with Russia’s top diplomat to the US.

Trump tweet: “FBI director Comey: fmr DNI Clapper ‘right’ to say no evidence of collusion between Russia and Trump Campaign.”

Fact check: Trump leaves out an important nuance in this tweet about the former director of national intelligence, James Clapper. Comey agreed there was no evidence of collusion in a report prepared by the US intelligence community under Clapper, but that’s different than saying no such evidence has been discovered. The January 6 report did not take up the question of contacts between Trump associates and Russia. Instead it focuses on Russia’s actions, via cyber operations and propaganda, to try to help Trump’s election chances and hurt Hillary Clinton’s.

Clapper indeed said he had seen no evidence of collusion at the time he left government. But he updated that comment on Monday with a statement from spokesman Shawn Turner, saying Clapper “could not account for intelligence or evidence that may have been gathered since the inauguration on January 20th”.

Comey: When asked if he was aware of collusion between Trump associates and Russia, beyond the findings of the report: “It’s not something I can comment on.”

Rogers: “Likewise, I’m not going to comment on an ongoing investigation’s conclusions.”

Additional reporting by Associated Press