A former FBI deputy director has said that a "crime may have been committed" when Donald Trump sacked the head of the organisation and tried to publicly undermine an investigation into his campaign's ties to Russia.
Andrew McCabe also said in the interview with CBS' 60 Minutes that the FBI had good reason to open a counterintelligence investigation into whether Mr Trump was in league with Russia, and therefore a possible national security threat, following the May 2017 firing of then-FBI director James Comey.
"And the idea is, if the president committed obstruction of justice, sacked the director of the of the FBI to negatively impact or to shut down our investigation of Russia's malign activity and possibly in support of his campaign, as a counterintelligence investigator you have to ask yourself, 'Why would a president of the United States do that?'" Mr McCabe said.
He added: "So all those same sorts of facts cause us to wonder is there an inappropriate relationship, a connection between this president and our most fearsome enemy, the government of Russia?"
Asked whether deputy attorney general Rod Rosenstein was on board with the obstruction and counterintelligence investigations, Mr McCabe replied: "Absolutely."
A Justice Department spokeswoman declined to comment on Sunday night.
Mr McCabe also revealed that when Mr Trump told Mr Rosenstein to put in writing his concerns with Mr Comey - a document the White House initially held up as justification for his firing - the president explicitly asked the Justice Department official to reference Russia in the memo.
Mr Rosenstein did not want to, Mr McCabe said, and the memo that was made public upon Mr Comey's dismissal did not mention Russia and focused instead on Mr Comey's handling of the Hillary Clinton email server investigation.
"He explained to the president that he did not need Russia in his memo," Mr McCabe said. "And the president responded, "I understand that, I am asking you to put Russia in the memo anyway."
Mr Trump said in a TV interview days after Mr Comey's firing that he was thinking of "this Russia thing" when he sacked Mr Comey.
Those actions, including a separate request by Mr Trump that the FBI end an investigation into his first national adviser, Michael Flynn, made the FBI concerned that the president was illegally trying to obstruct the Russia probe.
Mr McCabe was sacked from the Justice Department last year after being accused of misleading investigators during an internal probe into a news media disclosure.
The allegation was referred to the US Attorney's office in Washington for possible prosecution, but no charges have been brought.
Mr McCabe has denied having intentionally lied and said on Sunday that he believes his sacking was politically motivated.
"I believe I was fired because I opened a case against the president of the United States," he said.
In excerpts released last week by CBS News, Mr McCabe also described a conversation in which Mr Rosenstein had broached the idea of invoking the Constitution's 25th Amendment to remove Mr Trump from office.
The Justice Department said in a statement that Mr Rosenstein, based on his dealings with Mr Trump, does not see cause to seek the removal of the president.
On Sunday Senator Elizabeth Warren, a Massachusetts Democrat who is seeking her party's nomination for president, told reporters that if the people around Mr Trump believe he cannot fulfil the obligations of his office, then they have a duty to invoke the 25th Amendment.
A favourite target of the president's ire, Ms Warren said she has no special knowledge on whether there are grounds to remove Mr Trump from office but said that "there are a whole lot of people who do see him every day who evidently were talking about invoking the 25th Amendment".