Here is more from CBS News:
Attorney General Merrick Garland has assigned the U.S. attorney in Chicago to review documents marked classified that were found at the Penn Biden Center for Diplomacy and Global Engagement in Washington, two sources with knowledge of the inquiry told CBS News. The roughly 10 documents are from President Biden's vice-presidential office at the center, the sources said. CBS News has learned the FBI is also involved in the U.S. attorney's inquiry.The material was identified by personal attorneys for Mr. Biden on Nov. 2, just before the midterm elections, Richard Sauber, special counsel to the president confirmed. The documents were discovered when Mr. Biden's personal attorneys "were packing files housed in a locked closet to prepare to vacate office space at the Penn Biden Center in Washington, D.C.," Sauber said in a statement to CBS News. The documents were contained in a folder that was in a box with other unclassified papers, the sources said. The sources revealed neither what the documents contain nor their level of classification. A source familiar with the matter told CBS News the documents did not contain nuclear secrets.
As noted by CBS, the Penn Biden Center is a think tank about a mile from the White House that is affiliated with the University of Pennsylvania and named for the sitting president.
Biden kept an office at the think tank after he left the vice presidency in 2017 until shortly before he launched his 2020 presidential campaign in 2019.