Former US president Donald Trump's lawyers on Thursday were asked by a federal judge to support claims that the FBI planted evidence as part of a case concerning classified documents found at his home.
Mr Trump and several parties have claimed that the FBI lied about items they obtained during an August 8 search of his Mar-a-Lago home in Florida, as part of a Department of Justice investigation into possible breaches of federal laws.
He made the claim as recently as Wednesday evening during a Fox News interview.
"Did they drop anything into those piles, or did they do it later?" Mr Trump asked.
Judge Raymond Dearie, who is serving as an independent special master in the case, is now requiring that Mr Trump's lawyers send in a sworn declaration by September 30, stating whether they believe files were planted.
He is also asking them to state whether they believe the FBI lied about where they obtained items listed in their search inventory.
Mr Dearie, recommended by Mr Trump's team and assigned by a Trump-appointed judge, will review thousands of documents the FBI found at Mar-a-Lago by the end of November.
His request appears to be the first time the legal team has been asked to support these claims, which have all been made outside court, and could put them in a tough spot if they are unsubstantiated.
"This submission shall be [Mr Trump's] final opportunity to raise any factual dispute as to the completeness and accuracy of the detailed property inventory," the special master wrote in his filing.
It is not the first time Mr Dearie has challenged claims made without evidence.
In a hearing this week, he pressed the lawyers as to whether Mr Trump declassified the sensitive government files the FBI found.
After telling the special master that they would expand on declassification claims if they viewed the documents, Mr Dearie said: "You can't have your cake and eat it too."