The Justice Department is asking a federal court to unseal the warrant the FBI used to search the Mar-a-Lago estate of former President Donald Trump, Attorney General Merrick Garland said Thursday, acknowledging extraordinary public interest in the case about classified records.

The request is striking because such documents traditionally remain sealed during a pending investigation. But the Justice Department appeared to recognise that its silence since the search had created a vacuum for bitter verbal attacks by Trump and his allies, and that the public was entitled to the FBI's side about what prompted action at the former president's home.

"The public's clear and powerful interest in understanding what occurred under these circumstances weighs heavily in favor of unsealing," said a motion filed in federal court in Florida.

Should the warrant be released — the request is now with the judge, and Trump can object — it could disclose unflattering information about the former president and about FBI scrutiny of his handling of sensitive government documents right as he prepares for another run for the White House. During his successful 2016 campaign, he pointed frequently to an FBI investigation into his Democratic opponent, Hillary Clinton, over whether she mishandled classified information.

It's unclear at this point how much information would be included in the documents, if made public, or if they would encompass an FBI affidavit that would presumably lay out a detailed factual basis for the search. The department specifically requested the unsealing of the warrant as well as a property receipt listing the items that were seized, along with two unspecified attachments.

An aerial view of former President Donald Trump's Mar-a-Lago estate in Palm Beach, Florida. Photo / AP

To obtain a search warrant, federal authorities must prove to a judge that probable cause exists to believe that a crime was committed. Garland said he personally approved the warrant, a decision he said the department did not take lightly given that standard practice where possible is to select less intrusive tactics than a search of one's home.

Neither Trump nor the FBI has said anything about what documents the FBI might have recovered, or what precisely agents were looking for. But the former president complained anew about the search.

Trump, who for years has lambasted the FBI and sought to sow distrust among his supporters in its decisions, said the warrant was served and the search conducted despite his cooperation with the Justice Department over the search.

In a post to his Truth Social platform, Trump said that his "attorneys and representatives were cooperating fully" prior to the search, and that government officials "could have had whatever they wanted, whenever they wanted, if we had it".

The Justice Department has until this afternoon to alert the judge about whether Trump will object to the release.

FBI and Justice Department policy cautions against discussing ongoing investigations, both to protect the integrity of probes and to avoid unfairly maligning someone who is being scrutinised but winds up ultimately not being charged. That's especially true in the case of search warrants, where supporting court papers are routinely kept secret as the investigation proceeds.

In this case, though, Garland cited the fact that Trump himself had provided the first public confirmation of the FBI search, "as is his right". The Justice Department, in its new filing, also said that disclosing information about it now would not harm the court's functions.

Attorney General Merrick Garland speaks at the Justice Department in Washington. Photo / AP

Even so, Garland, in a hastily scheduled public statement delivered from the Justice Department podium, appeared to acknowledge the unusual nature of the department's request as he declined to take questions or provide any substantive details about the FBI's investigation.

"Much of our work is by necessity conducted out of the public eye. We do that to protect the constitutional rights of all Americans and to protect the integrity of our investigations," he said. "Federal law, longstanding department rules and our ethical obligations prevent me from providing further details as to the basis of the search at this time."

The Justice Department under Garland has been leery of public statements about politically charged investigations, or of confirming to what extent it might be investigating Trump as part of a broader probe into the January 6 riot at the US Capitol and efforts to overturn the results of the 2020 election.

The department has tried to avoid being seen as injecting itself into presidential politics, as happened in 2016 when then-FBI director James Comey made an unusual public statement announcing that the FBI would not be recommending criminal charges against Clinton regarding her handling of email — and when he spoke up again just over a week before the election to notify Congress that the probe was being effectively reopened because of the discovery of new emails.

The Mar-a-Lago search warrant was served Monday and was part of an ongoing Justice Department investigation into the discovery of classified White House records recovered from Trump's home in Palm Beach, Florida, earlier this year. The National Archives had asked the department to investigate after saying that classified records were found in 15 boxes of records that it retrieved from the estate. Multiple federal laws govern the handling of classified information.

The attorney general condemned verbal attacks on FBI and Justice Department personnel over the search. Some Republican allies of Trump have called for the FBI to be defunded, and large numbers of Trump supporters have called for the warrant to be released because they believe it will show that Trump was wrongly targeted.

"I will not stand by silently when their integrity is unfairly attacked," Garland said of federal law enforcement agents, calling them "dedicated, patriotic public servants".

Earlier Thursday, an armed man wearing body armour tried to breach a security screening area at an FBI field office in Ohio, then fled and was later killed after a standoff with law enforcement.

Associated Press writers Zeke Miller, Lindsay Whitehurst and Meg Kinnard contributed to this report.

- Eric Tucker and Michael Balsamo, AP