President Donald Trump says he will demand an inquiry into whether his election campaign was infiltrated for political purposes.
In a tweet, Mr Trump said he wanted to know whether his predecessor’s administration ordered such a move.
The request, which will be made officially on Monday, comes after US media reports suggesting the FBI had an informant meeting campaign aides.
There is already an investigation into all aspects relating to the campaign.
Mr Trump’s demand came amid a series of tweets on Sunday denouncing a “witch hunt” that, he said, had found no collusion with Russia.
This refers to the ongoing investigation led by Special Counsel Robert Mueller into Russia’s efforts to interfere in the 2016 US election, whether there was any collusion between the Kremlin and Mr Trump’s election campaign and whether the president tried unlawfully to obstruct the inquiry.
Mr Trump has constantly attacked the inquiry.
Was there a mole inside the Trump team?
Mr Trump first made the accusation that the FBI had sent a spy into his campaign team on Friday.
“It took place very early on, and long before the phony Russia Hoax became a ‘hot’ Fake News story,” Mr Trump tweeted, adding: “If true – all time biggest political scandal!”
The New York Times followed with an article that suggested there was, indeed, and FBI informant – whose identity it did not reveal – who had been sent to speak to campaign aides but only after the FBI had received reports of “suspicious contacts linked to Russia”.
The informant – an American academic working in the UK – had made contact with George Papadopoulos and Carter Page.
The Washington Post reported a similar account.
What’s likely to happen next?
Law enforcement officials have refused to provide evidence to Congressional leaders over the issue.
They have argued that doing so would put the informant’s life – or that of his contacts – in danger.
The president’s latest intervention raises the stakes further.
Mr Trump could order the Department of Justice – which has oversight over the FBI – to release the documents.
But analysts say this could pave the way for a collision with top officials.
What is the informant’s connection to Mueller?
According to the Washington Post, the informant has been aiding the Russia investigation since before Mr Mueller’s appointment a year ago. The FBI opened the inquiry in the middle of the election campaign in July 2016.
But it remains unclear how the informant first came by the information that led to his meetings with Mr Papadopoulos and Mr Page – and his wider role as an FBI informant.
As for Mr Mueller, the former FBI head has so far charged 19 people. Mr Papadopoulos has pleaded guilty to lying to the FBI about the timing of meetings with alleged go-betweens for Russia.
But Mr Trump and his supporters have multiplied their attacks on the special counsel’s work.
Without providing any evidence on Sunday, Mr Trump demanded a stop to the inquiry – that he said was nearing $20m in costs and was composed of 13 “angry and heavily conflicted Democrats”. Mr Mueller is a Republican.