(NewsNation) — The FBI executed a search warrant Monday at former President Donald Trump’s Mar-a-Lago resort in Florida, which sources confirmed to NewsNation was related to an investigation into whether Trump mishandled presidential records.

The Department of Justice has been investigating the discovery of boxes of White House records, some of them classified, that were taken to Mar-a-Lago after Trump left office. Sources confirmed to NewsNation that the search Monday was related to that probe.

A source close to the investigation told NewsNation they left with “dozens” of boxes.

“Nothing like this has ever happened to a President of the United States before,” Trump said in a statement about the FBI’s search. “After working and cooperating with the relevant Government agencies, this unannounced raid on my home was not necessary or appropriate.”

According to sources, the search lasted from 9 a.m. to 6:30 p.m. local time. The FBI went into the resort wearing street clothes and only told the Secret Service they were coming an hour before they arrived.

Former FBI agent Jennifer Coffindaffer called the search “hugely significant.”

“This means they have current probable cause to believe there is evidence of a crime at his estate,” she said Monday on “NewsNation Prime.”

A source told NewsNation the warrant only allowed agents to remove items related to the search; they did not have carte blanche to remove anything and sort it out later. Three lawyers for the former president showed up during the 9 1/2-hour search, a source said.

Trump compared the search of Mar-a-Lago to the Watergate break-in and said America has become a third-world country, “corrupt at a level not seen before.” He said agents broke into a safe in his office, though a source told NewsNation nothing was found inside.

“If they were able to search a safe, that tells me the specific items listed could be in a place as small as a safe,” Coffindaffer said.

The Department of Justice and FBI both declined to immediately comment on the search.

The discovery of classified information at Mar-a-Lago was referred to the Justice Department by the National Archives and Records Administration, which said earlier this year it had found classified material in 15 boxes at the residence.

Federal law bars the removal of classified documents to unauthorized locations, though it is possible that Trump could try to argue that, as president, he was the ultimate declassification authority.

A person familiar with the matter, who spoke on condition of anonymity to the Associated Press, said agents were also looking to see if Trump had additional presidential records or any classified documents at the estate.

The federal search of a former president’s home is unprecedented, and Trump called Monday’s action “prosecutorial misconduct.”

The Justice Department is also investigating efforts by Trump and his allies to undo the results of the 2020 election and any role he might have had in the Jan. 6, 2021, Capitol riots.

Attorney General Merrick Garland has been under pressure from some congressional Democrats to bring charges against Trump. Garland has previously said the department will hold anyone accountable, no matter their position if they break the law.

House Republican Leader Kevin McCarthy denounced the search, saying the Justice Department has “reached an intolerable state of weaponized politicization.”

“When Republicans take back the House, we will conduct immediate oversight of this department, follow the facts, and leave no stone unturned,” McCarthy tweeted. “Attorney General Garland, preserve your documents and clear your calendar.”

The Associated Press contributed to this report.