The chairman of the US Senate Judiciary Committee has vowed to get to the bottom of allegations that discussions were held in 2017 on removing President Donald Trump from office.
Senator Lindsey Graham pledged to issue subpoenas “if that’s what it takes”.
Ex-acting FBI chief Andrew McCabe has said deputy US attorney general Rod Rosenstein discussed the numbers needed to invoke the 25th Amendment.
Mr Rosenstein has in the past denied discussing invoking the clause.
The amendment provides for the removal of a president if deemed unfit.
How have these allegations arisen again?
They are certainly not new.
They have returned to the spotlight on Sunday, with Mr McCabe appearing on the 60 Minutes show on CBS in the US.
In comments released ahead of the airing of the show, he details what he says Mr Rosenstein discussed as regards the 25th Amendment.
Mr McCabe says: “The discussion of the 25th Amendment was simply [that] Rod raised the issue and discussed it with me in the context of thinking about how many other cabinet officials might support such an effort.
“The deputy attorney general was definitely very concerned about the president, about his capacity and about his intent at that point in time.”
He restates his allegation that Mr Rosenstein had considered wearing a wire in meetings with Mr Trump.
Mr McCabe took over the FBI in 2017 after Mr Trump fired James Comey amid tension over the investigation into alleged collusion between his campaign team and Russia during the 2016 presidential election.
Mr McCabe was himself fired as deputy director in March last year just two days before he was due to retire. He has now written a book on his time in the post.
On the Russia inquiry, Mr McCabe says he was “very concerned that I was able to put the Russia case on absolutely solid ground in an indelible fashion that were I removed quickly and reassigned or fired that the case could not be closed or vanish in the night without a trace”.
How did Senator Graham respond?
The Republican was interviewed on CBS on Sunday morning, after some of Mr McCabe’s comments were released early.
“It’s stunning to me that one of the chief law enforcement officers of the land would go on national television and say, oh by the way I remember a conversation with the deputy attorney general about trying to find if we could replace the president under the 25th Amendment,” Mr Graham said.
“I think everybody in the country needs to know if it happened. I’m going to do everything I can to get to the bottom of Department of Justice [and] FBI behaviour toward President Trump and his campaign.”
He pledged to hold a hearing to determine “who’s telling the truth”.
What has Mr Rosenstein previously said?
Last September he strongly denied discussing invoking the constitutional clause to oust President Trump.
America’s second most senior law official said the allegation was “inaccurate and factually incorrect”.
He said: “Let me be clear about this: Based on my personal dealings with the president, there is no basis to invoke the 25th Amendment.”
A source told the BBC that the comment that Mr Rosenstein was thinking of secretly recording Mr Trump was sarcastic.
In January, US media reported that Mr Rosenstein was planning to quit, although no timeframe had been set.
What is the 25th Amendment?
It provides for the removal of a president if he is deemed unfit for office. Duties are transferred to the vice-president.
Activating the relevant section of the 25th Amendment would require the approval of eight of the 15 members of Mr Trump’s cabinet, the vice-president and two-thirds majorities in Congress.
Ronald Reagan and George W Bush used the amendment to temporarily transfer power when they were medically anaesthetised.