Total number of Biden documents known to be marked classified about 20

The approximately 10 documents marked classified and discovered at the Penn Biden Center included top-secret material, according to a federal law enforcement official familiar with the investigation.

Top secret is the highest of the three basic levels of classification: confidential, secret and top secret. A leak of top secret information could cause "exceptionally grave damage," CBS reports.

Fewer than 10 documents marked classified were found at the Biden residence in Wilmington, Del., and none were marked top secret.

In all, the source said, the total number of known documents marked classified is roughly 20, between the two locations. 

CBS News reported Monday that roughly 10 documents with classified markings had been found at the Penn Biden Center for Diplomacy and Global Engagement in Washington. The documents were from President Biden's vice-presidential office at the center.   

Then on Thursday, the White House confirmed that a second group of records with classified markings dating from Mr. Biden's time as vice president had been found in his home in Wilmington, Delaware. 

On the same day, Attorney General Merrick Garland named a special counsel, Robert Hur, to oversee the investigation into the documents.