They were arguably Donald Trump's most striking claims to date: a series of tweets, accusing then-President Barack Obama of wire-tapping Trump Tower during the election.
The claim appeared to come as a surprise, and was not backed up by any evidence.
First Mr Obama's spokesman, then the former US intelligence chief James Clapper denied any wiretap had been ordered.
But President Trump, via his press secretary Sean Spicer, has urged Congress to investigate the "troubling" claims, alongside its current investigations into allegations of Russian hacking during the election.
What are the Congressional committees already looking at?
Both the House and Senate intelligence committees are currently looking into the possibility of Russian interference during the 2016 election, inquiries launched in January.
They have promised wide-ranging investigations, carried out on a bipartisan basis, which will not shy away from looking into potential links between the Trump campaign and the Kremlin, as well as Russian "cyber activity".
Both committees have conceded these will be time-consuming investigations, and at least one member of the House intelligence committee has expressed his annoyance at another layer being added
"Ok @PressSec, as a member of the committee onto which you've dumped this mess, I look forward to seeing your evidence," Democrat Jim Himes tweeted.
Is there any evidence?
Mr Trump and his team have provided no evidence to say the phones at Trump Tower were wiretapped, and now Mr Clapper, who was then director of National Intelligence, has categorically denied such an order existed.
As intelligence director, he told NBC, he would have known about a "Fisa court order on something like this".Media captionJames Clapper says he "can deny" that Donald Trump's phones were tapped
Could Obama have ordered the surveillance?
Mr Obama's team have outright denied any involvement, saying Mr Trump's tweets are "simply false".
A warrant, if it existed, would most likely have been ordered by the Department of Justice independently of the White House.
On Sunday, former press secretary Josh Earnest told ABC: "This may come as a surprise to the current occupant of the Oval Office, but the president of the United States does not have the authority to unilaterally order the wiretapping of an American citizen.
"If the FBI decided to use their wiretapping authority in the context of the counterintelligence or criminal investigation, it would require FBI investigators, officials at the Department of Justice going to a federal judge, and making a case, and demonstrating probable cause to use that authority to conduct the investigation. That is a fact."Image copyright AP Image caption It is unclear whether the claims have any basis in fact
The only way Mr Obama could have ordered surveillance without going through the Foreign Intelligence Court (Fisa court) is if there was no US citizens involved.
In this case, considering the target is allegedly Trump Towers - which definitely involves American citizens - this would have been hard to argue.
What is the Fisa court?
The Fisa court has been described by CNN as possibly "the most powerful court you have never heard of".
The secretive court approves surveillance warrants under Fisa against "agents of a foreign power", mainly to either the FBI or NSA.
In this case, the reported target of the FBI's application was two Russian banks.
Any application has to be approved by 11 sitting judges.
Where did the wiretap claim come from?
Rumours of an order granted under the Foreign Intelligence Surveillance Act (Fisa) have been circulating for some time.
On 7 November - the day before the country went to the polls - former British MP Louise Mensch reported the order was approved in October..
According to Ms Mensch, it was issued in connection with a private server and activity between two Russia banks.
She also mentions - like Mr Trump in his tweets - that an earlier order had been turned down.
In January, the BBC's Paul Wood - writing in the wake of claims the Russian government had compromising material on Mr Trump - said he also knew of the Fisa order.
Neither Mr Trump nor his associates are mentioned in the order, Mr Wood reported.
The Guardian also mentions the order at about the same time.
None of them mention wiretapping of phones.
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It is then seemingly not mentioned again until Mark Levin, a conservative radio host in the US, talked about it on Thursday, when he referred to it as a "silent coup".
His show was followed up with an article in Breitbart on Friday, which mentions wiretapping and "eavesdropping" sought by the "Obama administration". It is not clear how their claims were sourced.
None of this has ever been confirmed by anyone in authority.
President Trump's tweets were sent out in a flurry the next morning. It is unclear if he was aware of any of the previous reports - he does not appear to have tweeted about it before.