Mr. Trump and his lawyers resisted efforts by the National Archives

Investigators for the Justice Department on Friday seized more than a half-dozen documents, some of them classified, at President Biden’s residence in Wilmington, Del., after conducting a 13-hour search of the home, the president’s personal lawyer said Saturday evening.

The remarkable search of a sitting president’s home by federal agents — at the invitation of Mr. Biden’s lawyers — dramatically escalated the legal and political situation for the president, the latest in a series of discoveries that has already led to a special counsel investigation.

During Friday’s search, six more items with classified markings — including some documents from his time as a senator and others from his time as vice president — were taken by investigators, along with surrounding materials, according to the statement from Bob Bauer, Mr. Biden’s attorney.

Mr. Bauer did not indicate what had prompted the search, saying only that the president’s lawyers had offered to provide access for a search “in the interest of moving the process forward as expeditiously as possible.” Justice Department investigators coordinated the search with Mr. Biden’s lawyers in advance, Mr. Bauer said, and the president’s personal and White House lawyers were present at the time.

Investigators Seize More Classified Documents From Biden’s Home A team from the Justice Department conducted a 13-hour search of the president’s Wilmington residence on Friday.

“The F.B.I. on Friday executed a planned, consensual search of the president’s residence in Wilmington,” said Joseph D. Fitzpatrick, an assistant U.S. attorney in Illinois who is serving as a spokesman for the special counsel investigating the Biden documents case.

The search agreement with Mr. Biden’s legal team was negotiated by John R. Lausch, a federal prosecutor picked to lead the initial inquiry last year. His replacement, Robert K. Hur, who was appointed to serve as the permanent special counsel in the case earlier this month, is expected to take over “shortly,” Mr. Fitzpatrick said.

Mr. Bauer said the Justice Department had requested that the search not be made public before it was conducted, “in accordance with its standard procedures, and we agreed to cooperate.” He did not provide any more detail about the nature of the documents that were taken or what level of classification had been stamped on them.

The search underscored the seriousness of the investigation into Mr. Biden’s handling of documents and, while not a surprise raid, in some ways resembled the extensive search of former President Donald J. Trump’s Mar-a-Lago estate in Florida last summer, with agents looking for classified documents they believed were in Mr. Trump’s possession.

Mr. Biden and his aides have repeatedly argued that the two cases are very different because the president has cooperated fully with authorities, while Mr. Trump and his lawyers resisted efforts by the National Archives and the Justice Department to return documents.

Mr. Trump — who at one point claimed that he had declassified all the documents in question just by thinking about doing so — and his advisers are also under investigation for obstructing the inquiry into the classified information.

Understand the Biden Documents Case The discovery at two locations of classified documents from President Biden’s time as vice president has prompted the Justice Department to scrutinize the situation. In Washington: Attorney General Merrick B. Garland’s appointment of a special counsel to investigate the situation drew a mixed reception from Republicans, who had hoped to spearhead the effort themselves. Two Cases in the Spotlight: Two presidents — Mr. Biden and former President Donald J. Trump — are now under investigation by special prosecutors for how they’ve handled classified documents. Here is how their cases compare. Last Days as Vice President: The appointment of a special counsel has focused new attention on the frenetic final stretch of Mr. Biden’s vice presidency in January 2017. Presidents and Their Prosecutors: Since the dark days of Watergate, every president but one has faced a special prosecutor looking into them or their associates.

But since the discovery of Mr. Biden’s documents, Mr. Trump has complained that Justice Department investigators were treating his successor differently.

“When is the F.B.I. going to raid the many homes of Joe Biden, perhaps even the White House?” Mr. Trump wrote in a statement on his social media site earlier this month.

The results of Friday’s search follow a series of discoveries by the president’s own lawyers of classified documents at the president’s Wilmington home and the Washington office Mr. Biden used before moving into the White House. The lawyers quickly turned the documents over to the National Archives and, later, to the Justice Department.

Mr. Biden did not reveal the discovery of some of those documents for nearly two months, after initially finding them on Nov. 2. He has said that it would eventually be revealed that he did nothing wrong.

“There is no there there,” Mr. Biden told reporters on Thursday evening during a trip to California.

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Regarding Friday’s search, Mr. Bauer said in his statement on Saturday: “Yesterday, D.O.J. completed a thorough search of all the materials in the president’s Wilmington home. It began at approximately 9:45 a.m. and concluded at around 10:30 p.m. and covered all working, living and storage spaces in the home.”

“D.O.J. had full access to the president’s home, including personally handwritten notes, files, papers, binders, memorabilia, to-do lists, schedules, and reminders going back decades,” he added, referring to the Justice Department.

Mr. Bauer did not make clear in his statement where in the Wilmington home the documents had been found. The previous classified documents were found in the home’s garage and in a nearby storage space.

Another statement, from Richard Sauber, a member of the White House Counsel’s Office, said that the search had been conducted and finished at the home on Friday, and neither the president nor Jill Biden, the first lady, was at the residence at the time.

On Friday evening, even as investigators were still going through his home, Mr. Biden traveled to Rehoboth Beach, where he owns another house, to spend the weekend there. The president’s lawyers have said they searched the Rehoboth Beach home earlier this month and found no relevant documents. Officials have not said whether Justice Department investigators plan to conduct another search of the property.

In his statement on Saturday, Mr. Bauer said that the president’s cooperation with investigators was evidence that Mr. Biden and the White House were acting in good faith.

“We have attempted to balance the importance of public transparency where appropriate with the established norms and limitations necessary to protect the investigation’s integrity,” he wrote. “We will continue to do so throughout the course of our cooperation with D.O.J.”

News of the lengthy search, and the discovery of more classified materials, is certain to provide new ammunition to the president’s critics, including Republican members of the House, who have already demanded information about the documents and their potential impact on national security.

In a letter to Ron Klain, the White House chief of staff, this past week, Representative James R. Comer, Republican of Kentucky, demanded that the president and his lawyers provide more information to Congress.

“It is troubling that classified documents have been improperly stored at the home of President Biden for at least six years, raising questions about who may have reviewed or had access to classified information,” Mr. Comer wrote.

On Saturday, after news of the latest discovery was reported, Mr. Comer tweeted: “Biden’s White House claimed all classified documents were turned over. Now the Justice Department found more. Is the scavenger hunt over? Americans need answers now.”

Mr. Comer, the chairman of the House Oversight Committee, has not demanded similar transparency from Mr. Trump regarding the classified documents found at his home.

Friday’s search underscored the unprecedented nature of the twin investigations being conducted by separate special counsels — one of the sitting president and one of his immediate predecessor — for improper handling of highly sensitive government documents.

A spokeswoman for Attorney General Merrick B. Garland declined to comment when asked if he had been consulted in advance about the search. She referred all questions to a spokesman for Mr. Hur, the special counsel.

Special counsels have broad autonomy to search locations associated with an investigation, or even seek search warrants, without seeking permission to undertake major investigative steps. Under departmental regulations, an attorney general could block a move, but only if the counsel’s behavior was “inappropriate” or grossly misguided.

Nonetheless, Mr. Garland would almost certainly have been informed about a consensual search through a departmental procedure known as an “urgent report” intended to keep leadership informed of the progress of the probe, said Mary McCord, a former top official in the Justice Department’s national security division.

“In this case, it seems that all along there has been an intent to cooperate with the investigation, that much is clear,” said Ms. McCord, a law professor at Georgetown’s law school. “What we still don’t have is an explanation of how this material, some of which dates back to his days in the Senate, ended up there.”

Mr. Biden’s possession of classified documents was first revealed to the public earlier this month, by CBS News.

Since then, the president’s aides have struggled to contend with a series of rolling disclosures of additional document discoveries and legal developments that have raised the scrutiny on Mr. Biden’s handling of the sensitive material.

Karine Jean-Pierre, the White House press secretary, has been pressed repeatedly by reporters about the developing story during her daily press briefing. At one point, she told journalists that neither she nor her staff had been involved in discussions about what to tell the public and when.

Dyana Hansen Blog

Dyana Hansen Blog

Mr. Trump and his lawyers resisted efforts by the National Archives

Mr. Trump and his lawyers resisted efforts by the National Archives