Top secret information is purportedly included in some of the classified documents reportedly found in US President Joe Biden's private office.
According to CBS News, a US partner of the BBC, papers with the highest classified level were among roughly 10 files at a think group carrying his name.
At one of his Delaware residences, less than 10 classified papers were discovered, although none of them was top secret, according to CBS.
"Exceptionally terrible harm" might result from the disclosure of highly sensitive information.
In the US, there are three fundamental classification levels: confidential, secret, and top secret.
According to CBS, which cited federal law enforcement officials involved with the inquiry, some 20 secret files were ultimately found between the two sites.
The first batch of the files was discovered at Mr. Biden's old organization, the Penn Biden Center in Washington, DC, back in November, but press stories on them just started to surface last Monday.
Days later, Republicans barely took control of the US House of Representatives in the midterm elections.
On Thursday, it was revealed that Biden's staffers had discovered a second cache of documents on December 20 in a room and garage at his residence in Wilmington, Delaware.
On Friday, Mr. Biden disregarded inquiries from reporters about the escalating scandal as he welcomed Japanese Prime Minister Fumio Kishida at the White House.
A day earlier, Mr. Biden told reporters the papers were discovered locked in a garage next to his 1960s Chevrolet Corvette sports vehicle. Mr. Biden has previously called his predecessor Donald Trump's alleged handling of sensitive data "extremely reckless."
The White House has said that it is fully cooperating and that the materials were "accidentally misplaced."
Following the appointment of a special counsel to look into the situation by Attorney General Merrick Garland on Thursday, House Republicans opened their probe on Friday.
Republican Jim Jordan, chair of the House Judiciary Committee, questioned if the agency had "intentionally withheld this information from the public on the eve of the 2022 election" in a letter to the Department of Justice initiating the investigation.
The letter states that "our most senior executive branch law enforcement personnel ought to be transparent and accountable to the American people."
The deadline for submitting all papers about the disclosures and the selection of a special counsel is January 27.
Friday morning, the head of the House Oversight Committee sent a letter to the White House inquiring as to whether the president's son, Hunter Biden, had access to documents at his Wilmington home.
Republicans have also demanded that the White House make Mr. Biden's visitor logs public, but the White House has declined to indicate whether or not it would do so.
According to a count by the Associated Press, Mr. Biden has spent roughly 200 days, or more than a fourth of his administration, in his home state of Delaware. This includes a weekend visit to Wilmington.
The justice department is also looking into Mr. Trump because he kept more than 300 classified documents at his Florida mansion, Mar-a-Lago, some of which were seized by FBI agents last August while carrying out a search warrant. These documents included some marked with secret and top-secret designations.