Former President Donald Trump said Monday that the FBI had searched his Palm Beach, Fla., home and had broken open a safe — an account signaling a dramatic escalation in the various investigations into the final stages of his presidency.

The search, according to multiple people familiar with the investigation, appeared to be focused on material that Trump had brought with him to Mar-a-Lago, his private club and residence, when he left the White House. Those boxes contained many pages of classified documents, according to a person familiar with their contents.

Trump delayed returning 15 boxes of material requested by officials with the National Archives for many months, only doing so when there became a threat of action being taken to retrieve them.

The FBI would have needed to convince a judge that it had probable cause that a crime had been committed to get a search warrant, and proceeding with a search on a former president's home would almost surely have required signoff from top officials at the bureau and the Justice Department.

A spokesperson for the FBI declined to comment, and Justice Department officials did not initially respond to requests for comment.

Trump was in the New York area at the time of the raid.

Trump, who campaigned for president in 2016 criticizing Hillary Rodham Clinton's practice of maintaining a private e-mail server for government-related messages while she was secretary of state, was known throughout his term to rip up official material that was intended to be held for presidential archives. One person familiar with his habits said that included classified material that was shredded in his bedroom and elsewhere.

"After working and cooperating with the relevant Government agencies, this unannounced raid on my home was not necessary or appropriate," Trump said, maintaining it was an effort to stop him from running for president in 2024. "Such an assault could only take place in broken, Third-World Countries.

"They even broke into my safe!" he wrote. "What is the difference between this and Watergate, where operatives broke into the Democrat National Committee? Here, in reverse, Democrats broke into the home of the 45th President of the United States."

Trump did not share any details about what the FBI agents said they were searching for.

The search took place Monday morning, a person familiar with it said, although Trump claimed agents were still there many hours later.

The search was at least in part for whether any records remained at the club, the person familiar with the search said.

Aides to President Joe Biden said they were stunned by the development and had learned of it from Twitter.

The reported search came at a time when the Justice Department has also been stepping up questioning of former Trump aides who had been witnesses to discussion and planning in the White House of Trump's efforts to remain in office after his loss in the 2020 election.

Trump has been the focus of questions asked by federal prosecutors in connection with a scheme to send "fake" electors to Congress for the certification of the Electoral College.

Current FBI Director Christopher Wray was appointed by Trump.

The law governing the preservation of White House materials, the Presidential Records Act, lacks teeth, but criminal statutes can come into play, especially in the case of classified material.

Criminal codes, which carry jail time, prohibit anyone who "willfully injures or commits any depredation against any property of the United States" and anyone who "willfully and unlawfully conceals, removes, mutilates, obliterates or destroys" government documents.

Samuel Berger, a national security adviser to President Bill Clinton, pleaded guilty in 2015 to a misdemeanor charge for removing classified material from a government archive. In 2007, Donald Keyser, an Asia expert and former senior State Department official, was sentenced to prison after he confessed to keeping more than 3,000 sensitive documents — ranging from classified to top secret — in his basement.

In 1999, the CIA announced it had suspended the security clearance of its former director, John Deutch, after concluding that he had improperly handled national secrets on a desktop computer at his home.

In January, the archives retrieved 15 boxes that Trump had taken with him to Mar-a-Lago from the White House residence when his term ended. The boxes included material subject to the Presidential Records Act, which requires that all documents and records pertaining to official business be turned over to the archives.

The items in the boxes included documents, mementos, gifts and letters. The archives did not describe the classified material it found other than to say that it was "classified national security information."

Because the National Archives "identified classified information in the boxes," the agency "has been in communication with the Department of Justice," David Ferriero, the national archivist, told Congress at the time.

Federal prosecutors subsequently began a grand jury investigation, according to two people briefed on the matter. Prosecutors issued a subpoena earlier this year to the archives to obtain the boxes of classified documents, according to the two people familiar with the matter, who spoke on condition of anonymity because of the ongoing investigation.

Authorities also made interview requests to people who worked in the White House in the final days of Trump's presidency, according to one of the people.