The US Department of Justice is reviewing potentially classified documents from Joe Biden's time as vice president that were found by his personal lawyers at a Washington think tank, a White House lawyer said on Monday.
Almost 10 documents were found at Mr Biden's office at the Penn Biden Centre for Diplomacy and Global Engagement, CBS News reported earlier.
It said US Attorney General Merrick Garland had asked the US attorney in Chicago to review the classified documents, which were handed over to the National Archives.
The material was identified by Mr Biden's lawyers on November 2, days before the midterm elections, Richard Sauber, special counsel to the president, said on Monday.
Mr Biden had periodically used that office space from mid-2017 until the start of his 2020 presidential campaign.
The White House Counsel's Office notified the National Archives on the day the documents were discovered, Mr Sauber said, adding the National Archives took possession of the material on the following morning.
He said the documents were not the subject of any previous request or inquiry by the National Archives.
The documents were discovered when Mr Biden's personal lawyers "were packing files housed in a locked closet to prepare to vacate office space at the Penn Biden Centre in Washington, DC", Mr Sauber said.
He said the White House was co-operating with the Justice Department and the National Archives.
Mr Sauber did not mention the number of classified documents or what they contained or their level of classification. CBS News reported that they did not contain nuclear secrets.
CNN said the files included top secret documents, with a “sensitive compartmented information” label.
The Justice Department is also investigating former President Donald Trump's handling of highly sensitive classified documents that he kept at his Florida resort after leaving the White House in January 2021.
FBI agents carried out a court-approved search of Trump's Mar-a-Lago estate on August 8, 2022.
About 100 documents marked as classified were among thousands of records seized.
FBI raid on Trump's Mar-a-Lago estate – in pictures
Republicans will probably draw parallels between the two cases and press the Biden administration for accountability.
"When is the FBI going to raid the many homes of Joe Biden, perhaps even the White House? These documents were definitely not declassified," Mr Trump said in a post on his Truth Social late on Monday.
Reuters contributed to this report