The chairman of the Judicial Committee of the US Senate has pledged to get to the bottom of the allegations that talks have been held in 2017 on the removal of President Donald Trump.
Senator Lindsey Graham is committed to issuing subpoenas "if that's what it takes".
Former FBI Acting Chief Andrew McCabe said that US Deputy Attorney General Rod Rosenstein had discussed the numbers needed to invoke the 25th Amendment.
Mr. Rosenstein has denied in the past that he has discussed the invocation of the clause.
The amendment provides for the dismissal of a president when he is declared unfit.
How did these allegations arise again?
They are certainly not new.
They came back in the spotlight on Sunday, with McCabe appearing on the 60 Minutes show on CBS in the United States.
In comments published before the broadcast of the broadcast, he details the remarks of Mr. Rosenstein concerning the 25th amendment.
Mr. McCabe said, "The 25th Amendment was simply [that] Rod raised the issue and discussed it with me as part of the reflection on the number of other Cabinet members likely to support such an effort.
"The Deputy Attorney General was definitely very concerned about the President, his ability and his intention at the time."
He reiterated his allegation that Mr. Rosenstein would have considered carrying a thread in meetings with Mr. Trump.
Mr. McCabe took the leadership of the FBI in 2017 following the dismissal of James Comey by Mr. Trump while investigating the alleged collusion between his campaign team and Russia in the 2016 presidential election was a source of tension.
McCabe himself was fired as deputy director last March, just two days before his retirement. He has now written a book about his posting.
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With regard to the Russian inquiry, Mr. McCabe said he was "very concerned that I was able to put the case of Russia on an absolutely solid, indelible basis, night without trace".
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How did Senator Graham respond?
The Republican was interviewed by CBS Sunday morning after the early publication of some of McCabe's comments.
"It's amazing to me that one of the country's law enforcement officials went on national television and said, oh, by the way, I remember a conversation with the Deputy Attorney General on the question of whether we could replace the President under the Amendment, "said Graham.
"I think everyone in the country needs to know if this has happened, and I will do everything I can to get to the bottom of things at the Department of Justice. [and] FBI behavior towards President Trump and his campaign. "
He is committed to holding a hearing to determine "who is telling the truth".
What has Mr Rosenstein already said?
Last September, he firmly denied having discussed the invocation of the constitutional clause to overthrow President Trump.
The second US policy maker said the allegation was "inaccurate and factually inaccurate".
He said, "Let me be clear about this: on the basis of my personal relationship with the president, there is no basis for invoking the 25th Amendment."
A source told the BBC that the comment that Rosenstein planned to secretly record Mr. Trump was sarcastic.
In January, the US media announced that Rosenstein was planning to resign, although no timetable has been set.
What is the 25th amendment?
It provides for the dismissal of a president when he is declared unfit to hold a position. The tasks are transferred to the Vice President.
Activation of the relevant section of the 25th amendment would require the approval of eight of the 15 members of Mr. Trump's cabinet, vice-chair and a two-thirds majority of Congress.
Ronald Reagan and George W. Bush used the amendment to temporarily transfer power after medical anesthesia.