Comey was sacked on 9 May, and initially the White House cited his handling of the investigation into Hillary Clinton's private email server as the reason for his termination, releasing a memo authored by deputy attorney general Rod Rosenstein as a basis for this action.
Trump's young presidency has been battered by allegations - under investigation both by Congress and the FBI - that Russian Federation interfered to sway the 2016 election in his favour, in possible collusion with Trump's campaign team.
After earlier reports had indicated that Trump asked former FBI Director James Comey to close the agency's investigation into former national security adviser Michael Flynn, some lawmakers and legal experts said if the reports are true, Trump may have committed obstruction of justice, an impeachable offense.
In the days following Trump's firing of Comey on May 9 and before Mueller's appointment, the obstruction-of-justice investigation of the president began, according to people familiar with the matter. On Monday, a Trump friend said the president was considering dismissing Mueller though the White House later said he had no plans to do so.
It wasn't immediately clear who Trump was referring to in the tweet, but it was nearly certainly Deputy Attorney General Rod Rosenstein.
Trump, in a most revealing interview with NBC News anchor Lester Holt, had said he had the handling of "the Russian thing" in mind when he decided on his own to fire Comey.
The Post quoted five people briefed on the requests, saying those who agreed to be interviewed include national intelligence director Daniel Coats, Admiral Mike Rogers who heads the NSA, and his recently departed deputy, Richard Ledgett. Trump described such stories as witch hunt.
The US intelligence community says that Russian Federation played a role in the US presidential election held last November.
At the December meeting with [Russian Ambassador Sergey] Kislyak, Kushner suggested establishing a secure communications line between Trump officials and the Kremlin at a Russian diplomatic facility, according to USA officials who reviewed intelligence reports describing Kislyak's account.
Trump tweeted on Friday that he is being investigated for firing Comey, calling it a "witch hunt".
On Thursday, Russian President Vladimir Putin said Comey had presented no evidence to prove that Moscow meddled in the United States election, adding that Washington had tried to influence Russian elections "year after year".
Comey testified that not everyone on his Federal Bureau of Investigation team agreed he should.
Democratic Senator Dianne Feinstein, a Senate Intelligence Committee member, said she was "increasingly concerned" Trump would try to fire not only Mueller, but also Rosenstein. A recent Washington Post report said Trump is being investigated for possible obstruction of justice by Special Counsel Robert Mueller.
Through a spokesman, Trump's personal attorney, Marc Kasowitz, blasted the Post's report, though he stopped short of denying it. "Sad!" Mr. Trump said on Twitter.
Trump spoke to Coats and Rogers about the Russian Federation investigation, according to officials.
"What I did is I was going to fire Comey".
Trump hadn't publicly admitted he was personally under investigation until now. When Comey testified last week, he also gave the committee and Mueller a document that involved Comey taking notes of his communications with President Trump.
The Republican leader also lashed out at the Justice Department's number two, the man tasked with organizing the probe.