It is not clear if the search, which was done with the cooperation and knowledge of Mr. Biden’s legal team, uncovered any additional classified files.
WASHINGTON — The Federal Bureau of Investigation searched a think tank founded by President Biden in mid-November after his aides discovered a small cache of classified documents there earlier that month, according to a person familiar with the situation.
It is not clear if the previously undisclosed search, which was done with the cooperation of Mr. Biden’s lawyers after the first batch was discovered, turned up any additional files dating from Mr. Biden’s eight years as vice president.
On Dec. 20, Mr. Biden’s team discovered a second trove of government documents intermingled with personal and political memorabilia at the president’s house in Delaware. The F.B.I. searched that residence in January at the invitation of Mr. Biden’s lawyers.
Neither the White House nor Mr. Biden’s personal lawyer, Bob Bauer, immediately responded to a request for comment. A Justice Department spokesman also declined to comment.
The F.B.I. search of the think tank, the Penn Biden Center for Diplomacy and Global Engagement, was earlier reported by CBS News.
The development was the latest in a series of piecemeal revelations about the Biden team’s discovery of government documents, and his aides’ interactions with the federal law enforcement agencies investigating the matter.
Mr. Biden’s voluntary compliance with the Justice Department and the F.B.I. stands in stark contrast with the behavior of former President Donald J. Trump, who refused to turn over dozens of classified documents demanded by the National Archives for months. The Justice Department secured a search warrant that resulted in an F.B.I. search of his Mar-a-Lago estate in Florida in August.
But White House officials and Mr. Biden’s lawyers have been reluctant to disclose details of the matter, resulting in a dribble of news reports and prompting congressional Republicans to attack the White House.
And significant questions remain about the incomplete timeline put out by both the Justice Department and Mr. Biden’s team, including why the president’s lawyers took about six weeks to sift through boxes stored at his Delaware house for other documents.
Robert K. Hur, who was appointed by Attorney General Merrick B. Garland as a special counsel to oversee the Biden investigation, began work this week, an official with knowledge of the situation said. Mr. Hur replaced John R. Lausch Jr., the U.S. attorney in Chicago, who initiated the investigation into the matter at Mr. Garland’s request.
Mr. Biden modulated between sharp criticism of Mr. Trump’s cavalier treatment of documents and a more measured assessment of the logistical complexities of handling sensitive government papers, even before revelations about his own actions surfaced.
In August, shortly after the search of Mr. Trump’s resort at Mar-a-Lago, Mr. Biden acknowledged that handling classified materials was a byproduct of a president’s frenetic schedule and the round-the-clock demands of the job.
“I’m taking home with me today, today’s P.D.B.,” he told reporters as he boarded a helicopter in the South Lawn of the White House, referring to the highly classified President’s Daily Brief, the intelligence summary prepared each morning.
Mr. Biden added that he considered taking documents home acceptable, “depending on the circumstance,” but emphasized that he was careful to use a custom-built secure room in his house in Wilmington, Del., to review sensitive materials.