The documents dating to the Obama-Biden administration were found at Biden's residence.
US President Joe Biden said he is "cooperating fully" after the White House said Thursday that "a small number" of classified documents had been found at his private residence, prompting calls for a congressional investigation into the alleged mishandling of official papers.
The potentially embarrassing discovery for Biden came as authorities investigate a far bigger scandal involving his predecessor Donald Trump.
The documents dating to the Obama-Biden administration were found at Biden's residence in Wilmington, Delaware, after other similar papers were found at a Washington think tank where Biden used to have an office.
Former president Trump hoarded huge numbers of documents at his Florida residence after leaving the White House in 2021, with the FBI carting away some 11,000 papers after serving a search warrant in August.
Trump could face obstruction of justice charges.
But the discovery of papers at Biden's house is politically tricky for a president who touts his high ethical standards, and it could also complicate the Trump probe.
"People know I take classified documents and classified material seriously," Biden told reporters Thursday.
"We're cooperating fully (and) completely with the Justice Department's review. As part of that process, my lawyers reviewed other places where documents from my time as vice president were stored, and they finished the review last night.
"They discovered a small number of documents with classified markings in storage areas and file cabinets in my home and my personal library.
"The Department of Justice was immediately notified... So we're going see all this unfold," he added, declining to take shouted questions from the press.
- Stray documents -
Republican House Speaker Kevin McCarthy demanded that "Congress has to investigate this," pointing to the Justice Department's investigation of Trump.
"When is the FBI going to raid the many homes of Joe Biden, perhaps even the White House," Trump had earlier written on his social media site Truth Social.
Biden's team has said that if mistakes were made with his official papers, the administration has immediately acted to rectify any errors.
After the first batch of documents was discovered in Biden's former office at the Penn Biden Center think tank last November, lawyers turned them over to the National Archives, which handles all such materials, the White House counsel's office said.
Lawyers for Biden then scoured possible locations for any other stray documents.
In a bid to defuse accusations of political interference, Attorney General Merrick Garland has named a Chicago federal prosecutor appointed during the Trump administration to review the lost Biden documents.
That response differentiates starkly from Trump, who did not cooperate with authorities' repeated attempts to track down missing documents -- leading to the FBI entering his home with a search warrant.