Some of the classified documents and "surrounding materials" dated from Biden's tenure in the U.S. Senate, where he represented Delaware from 1973 to 2009, according to his lawyer, Bob Bauer.

Other classified government records were discovered this month at Biden's Wilmington residence, and in November at a private office he maintained at a Washington, D.C., think tank after ending his tenure as vice president in the Obama administration in 2017.

Special counsel Robert Hur, who was appointed during the process, is investigating how the president and his team handled Obama-era classified documents that were recently found in Biden's private possession.

The search escalates the legal and political stakes for the president, who has insisted that the previous discovery of classified material at his home and former office would eventually be deemed inconsequential.

Biden said on Thursday he has "no regrets" about not publicly disclosing before the midterm elections the discovery of classified documents at his former office and he believed the matter will be resolved.

Reporting by Nandita Bose, Matt Spetalnik, Steve Holland and Joel Schectman Editing by Nick Zieminski and David Gregorio