Lawyers for Mr Biden have said that in addition to an office cupboard at a think tank in Washington and a cardboard box in his garage in Wilmington, more pages of classified information had been found in his private library.
The discovery which was made on Thursday but announced on the weekend, piles more pressure on Mr Biden to explain what he was doing with documents including US intelligence memoranda and briefing materials on Ukraine, Iran and the United Kingdom.
A special counsel has been ordered to investigate, while at least two House Committees will open their own probes.
Discussion is rife in Democrat circles over whether he should now step aside to allow another candidate to run for the White House in 2024.
Mr Biden’s allies have reportedly pointed to distinctions between the Trump and Biden cases.
Fewer than a dozen classified documents were located at the Penn Biden Centre think tank. It is not known how many were found in Wilmington, but it is not thought to be a large number. When Mr Biden’s documents were discovered, he handed them over immediately. They had been, his lawyer said, "inadvertently misplaced’’.
In Trump’s case, the National Archives tried for more than a year after he left office to retrieve all of the records he retained, without success.
When he finally returned 15 boxes of documents in January 2022, archives officials found they contained classified materials.