The investigation, according to several people familiar with the investigation, appeared to focus on material Mr. Trump brought with him to Mar-a-Lago, his private club and residence, when he left the White House. Those boxes contained several pages of classified documents, according to a person familiar with their contents. Mr. Trump delayed returning 15 boxes of material requested by officials at the National Archives for months, only when there was a threat of action to retrieve them. The case was referred to the Department of Justice by the archives earlier this year. Former President Donald J. Trump said FBI agents searched Mar-a-Lago, his private club and residence in Florida, and broke a safe.Credit…MediaPunch, via Associated Press The investigation marked the latest notable turn in long-running investigations into Mr. Trump’s actions before, during and after his presidency — and even as he announces another run for the White House. It came as the Justice Department stepped up its separate investigation into Mr. Trump’s efforts to stay in office after his 2020 election defeat, and as the former president also faces an accelerated criminal investigation in Georgia and civil lawsuits in New York. . Mr. Trump has long viewed the FBI as a tool of Democrats to arrest him, and the investigation has sparked a furious backlash among his supporters in the Republican Party and on the far right of American politics. Representative Kevin McCarthy of California, the Republican leader in the House, has suggested he intends to investigate Attorney General Merrick B. Garland if Republicans take control of the House in November. The FBI would have to convince a judge that it had probable cause that a crime had been committed and that agents could find evidence at Mar-a-Lago to obtain a search warrant. Continuing an investigation into a former president’s home would almost certainly require a sign-off from top officials in the office and the Justice Department. The investigation, however, does not mean that prosecutors have found that Mr. Trump committed a crime. An FBI spokesman declined to comment, as did Justice Department officials. The director of the FBI, Christopher A. Wray, was appointed by Mr. Trump. Mr. Trump was in the New York area at the time of the investigation. “Another day in heaven,” he said Monday night during a phone rally for Sarah Palin, who is running for a congressional seat in Alaska. Eric Trump, one of his sons, told Fox News that he was the one who informed his father that the investigation was taking place and said the search warrant related to presidential documents. Mr Trump, who campaigned in 2016 criticizing Hillary Clinton’s practice of maintaining a private email server for government-related messages while she was secretary of state, has been known throughout his tenure to seize official material meant to be kept for presidential elections. archives. A person familiar with his habits said it included classified material being shredded in his bedroom and elsewhere. The investigation was at least partly about whether records remained at the club, said a person familiar with the matter. It happened Monday morning, the person said, although Mr. Trump said agents were still there several hours later. “After working and cooperating with the appropriate government agencies, this unannounced raid on my home was not necessary or appropriate,” Mr. Trump said, arguing that it was an attempt to prevent him from running for president in 2024. “A such an attack could only take place in devastated Third World countries.” “They even broke into my safe!” He wrote. Mr. Trump did not share details of what the FBI agents were looking for. Aides to President Biden said they were surprised by the development and learned about it on Twitter. The investigation comes as the Justice Department also ramps up its questioning of former Trump aides who witnessed White House discussions and planning of Mr. Trump’s efforts to overturn his election loss. Mr Trump has been at the center of questions raised by federal prosecutors over a plan to send “fake” electors to Congress to certify the Electoral College. The House committee investigating the Jan. 6 attack on Capitol Hill is also continuing its work and interviewing witnesses this week. The law governing the preservation of White House materials, the Presidential Records Act, has no teeth, but criminal statutes can come into play, especially in the case of classified material. The penal codes, which carry imprisonment, can be used to prosecute anyone who “willfully injures or commits any plunder of any property of the United States” and anyone who “willfully and unlawfully conceals, removes, mutilates, destroys, or destroys” government documents. Samuel R. Berger, national security adviser to President Bill Clinton, pleaded guilty in 2015 to a misdemeanor charge of removing classified material from a government file. In 2007, Donald Kizer, an Asia expert and former senior State Department official, was sentenced to prison after he confessed to keeping more than 3,000 sensitive documents – from classified to top secret – in his basement. In 1999, the CIA announced that it had suspended the security clearance of its former director, John M. Deutch, after concluding that he had improperly handled national secrets on a desktop computer at his home. In January this year, the archives recovered 15 boxes that Mr Trump took with him to Mar-a-Lago from the White House residence when his term ended. The boxes contained material subject to the Presidential Records Act, which requires all documents and records relating to official business to be turned over to the archives. Items in the boxes included documents, memorabilia, gifts and letters. The files did not describe the classified material he found other than that it was “classified national security information.” Because the National Archives “identified classified information in the boxes,” the agency “was in contact with the Department of Justice,” David S. Ferriero, the national archivist, told Congress at the time. Federal prosecutors subsequently launched a grand jury investigation, according to two people briefed on the matter. Prosecutors subpoenaed the records earlier this year 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 at the White House in the final days of Mr. Trump’s presidency, according to one of the people. In the spring, a small group of federal agents visited Mar-a-Lago looking for some documents, according to a person familiar with the meeting. At least one of the agents was involved in counterintelligence, according to the person. The question of how Mr. Trump handled sensitive material and documents he received as President has arisen throughout his time in the White House and beyond. He was known to tear up pieces of official paper given to him, forcing officials to tape them back up. And a forthcoming book by a New York Times reporter reveals that staff members would find wads of torn paper clogging a toilet and believed he had flushed them inside. The question of how Mr. Trump handled classified material is complicated because as president he had the power to declassify any government information. It is unclear whether Mr. Trump, before leaving office, had declassified materials discovered in the boxes. Under federal law, he no longer retains the ability to declassify documents after he leaves office. While in office, he invoked the authority to declassify information several times as his administration released material that helped him politically, particularly on issues such as the investigation into his campaign’s ties to Russia. Toward the end of the administration, Mr. Trump leaked photos that intrigued him from the President’s Daily Brief — a compendium of often classified information about potential national security threats — but it’s unclear whether he took them home with him. In a prime example of how he has dealt with classified material, Mr Trump in 2019 took a highly classified spy satellite image of an Iranian missile launch site, declassified it and then tweeted the photo. Earlier this year, Kash Patel, a former senior Defense Department official and Trump loyalist whom Mr. Trump named as one of his spokespeople to work with the National Archives, suggested to the right-wing website Breitbart that Mr. Trump had declassified the documents before leaving the White House and that the proper markings had simply not been adjusted. “Trump declassified entire sets of material in anticipation of leaving the administration that he believed the American public should have the right to read for themselves,” he said, according to Breitbart. Local television crews showed Mr. Trump supporters gathering near Mar-a-Lago, some of whom were aggressive toward reporters. Mr. Trump made it clear in his statement that he sees potential political value in the investigation, something echoed by some of his advisers, depending on what any investigation produces. His political team began sending out fundraising appeals about the investigation late Monday afternoon. Jonathan Martin, Luke Broadwater and Glenn Thrush contributed reporting.