Sources familiar with the matter said the second set of documents contained classified information. They came to light as Biden aides conducted an extensive search of locations where he worked after leaving the Obama administration.

Biden aides were responding to the November discovery of classified documents at the Washington, D.C., office of the University of Pennsylvania's Penn Biden Center, which Biden used after his time as vice president.

The original November discovery was immediately reported to the National Archives and Records Administration, according to White House officials, sparking a Department of Justice inquiry that became public this week.