The letter said the investigation will also probe Mr Comey's testimony that Loretta Lynch, as President Barack Obama's attorney general, had directed him to describe an FBI probe into Hillary Clinton's email practices as merely a "matter" and to avoid calling it an investigation. A couple of months ago, he told a congressional committee that a counterintelligence investigation into the Trump campaign was ongoing.
US Attorney-General Jeff Sessions has vehemently denied any collusion with Russian Federation to tilt the election in Mr Donald Trump's favour, branding the suggestion an "appalling and detestable lie".
At other times, though, Sessions frequently said he couldn't recall specifics, particularly when asked about his meetings with Russian officials during the 2016 campaign.
When asked to explain why he wrongly claimed in his confirmation hearing that he never met with Russians, Sessions said he was flustered by the question from Sen. Sessions reiterated Tuesday those meetings were in his capacity as a lawmaker and not about the campaign.
Sessions was adamant that he did not have a private meeting with Kislyak at that event.
"Prior to the speech I attended by the president, I attended a reception with my staff that included at least two dozen people and President Trump. So I would push back on that", Sessions said.
Democrats came at Sessions on a number of points, one big one being his dealings with Russian Ambassador Sergey Kislyak.
As even Comey stated last week, Trump is not a target of any investigation about Russia's alleged role in the election. Perhaps Trump will fire Mueller first, or maybe Trump will try and pardon himself, or he could just pull a Sarah Palin and just walk away from the job.
Can a president be convicted in a court of law for obstruction of justice for firing Comey, even though it was something he had a constitutional right to do and even though Comey's behavior during the 2016 campaign merited his dismissal?
He also explained that he had extensive meetings with ethics experts of the DOJ, who also recommended that he recuse himself from this specific investigation due to possible conflicts of interest. He claimed that his recusal had nothing to do with his own failure to disclose meetings with Mr. Kislyak - absurd, given the context when he made the decision - and flailed at trying to square his lack of involvement in Russian Federation matters with his role in firing Mr. Comey for his handling thereof.
Sessions also pushed back against Comey's suggestion he may have failed to take appropriate steps to protect the Federal Bureau of Investigation chief from political pressure.
A White House spokesman said later that Mr Trump "has no intention" of dismissing Mr Mueller.
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Even if it was meant as a sort of passing joke, that's a odd attitude to take when you're under oath at an intelligence committee hearing. So Sessions would not answer in order to, as he told Senator Angus King, "protect" Trump's possible future privilege assertions.
"I don't think it's for me to say whether the conversation I had with the president was an effort to obstruct. I am following the historic policies of the Department of Justice".
In his testimony on June 8, Comey told the Senate Intelligence Committee that he believed Trump had fired him over his role as lead of the FBI investigation into Russian interference in the US election and Trump campaign associates' possible ties to Russia.
Trump is not the only one under scrutiny, Barrett said: Investigators are also looking into the finances of Trump associates.