According to Mr. Biden's lawyer, US Department of Justice (DoJ) investigators allegedly found six additional classified documents during a 13-hour search of the president's Delaware home.
Papers from his tenure as a senator and as Barack Obama's president were discovered at the Wilmington residence on Friday.
Additionally taken out were "surrounding paperwork" and private handwritten notes, according to lawyer Bob Bauer.
Both Mr. Biden and his wife were not present.
The president had given "access to his house to allow DoJ to execute a search of the complete premises for probable vice-presidential papers and perhaps secret material," according to a statement from Mr. Bauer on Saturday.
According to Mr. Biden's counsel earlier this month, the first collection of sensitive information was found on November 2 at the Penn Biden Center, a think tank the president founded in Washington, DC.
According to his lawyers, a second set of papers were found in the garage of his Wilmington house on December 20, and a separate document was found in a storage space on January 12.
The president said that as soon as they learned about the papers, his staff immediately turned them over to the National Archives and the Justice Department. Why Mr. Biden still kept them is a mystery.
According to the Presidential Documents Act, when an administration is over, White House documents must be moved to the National Archives, where they may be maintained securely.
Robert Hur has been chosen as the special attorney to investigate how the private documents were handled.
The president views the lengthy investigation and subsequent discovery of more files as a political problem as he prepares to declare if he will seek reelection in 2024.
The Bidens are spending the weekend at Rehoboth Beach, Delaware, a beachside town where they also have a residence. According to The New York Times, a search was conducted earlier this month but no documents were found.
The two-month gap between the first Biden discovery, which happened just before the midterm elections, and the news being public in January, according to writer Anthony Zurcher, presents uncomfortable questions for the president regarding transparency.
The Biden campaign claims that the president provided full assistance to the DoJ probe. Mr. Biden has minimized the episode as an oversight and says he has "no regrets" about delaying the release of information about the discovery of specific classified papers until after the November midterm elections.
The discovery coincides with an inquiry into former US President Donald Trump's suspected handling of hundreds of sensitive documents at his Florida Mar-a-Lago resort and his alleged failure to appear as required.
In August of last year, the FBI conducted a raid on Mr. Trump's Florida vacation home, but he and his counsel were slow to give up the information. He claims that President Biden is receiving better treatment from the FBI.
President Biden referred to Mr. Trump's handling of the information at the time as "very irresponsible."
Legal authorities have refuted Mr. Trump's claim that he could declassify them, although he has not said why they are there.
