Classified files recovered from US President Joe Biden's former private office reportedly include some material marked top secret.
Papers with the highest classification level were among about 10 files at a think tank bearing his name, according to the BBC's US partner CBS News.
Fewer than 10 classified documents were found at one of his Delaware homes, but none were top secret, says CBS.
A leak of top secret information could cause "exceptionally grave damage".
There are three basic levels of US classification: confidential, secret and top secret.
In total, roughly 20 classified files were recovered between the two locations, reports CBS, citing a federal law enforcement official familiar with the investigation.
The papers only began to come to light in news reports last Monday, but the first batch was found at Mr Biden's former institute, the Penn Biden Center in Washington DC, back in November.
That was days before the midterm elections, which saw Republicans narrowly win control of the US House of Representatives.
On Thursday it emerged that a second cache of files had been found on 20 December by Biden aides in a garage and adjacent room at his private home in Wilmington, Delaware.
On Friday, as he hosted Japanese Prime Minister Fumio Kishida at the White House, Mr Biden ignored questions from journalists about the growing controversy.
A day earlier, Mr Biden - who has previously described his predecessor Donald Trump's alleged mishandling of classified material as "totally irresponsible" - told reporters the documents were found locked in a garage next to his 1960s Chevrolet Corvette sports car.
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