Nearly 10 documents were reportedly found at Washington's Penn Biden Center for Diplomacy and Global Engagement, an office which Biden periodically used from mid-2017 until the start of his 2020 presidential campaign.
U.S. Attorney General Merrick Garland had asked the U.S. attorney in Chicago - a Trump appointee - to review the material, which was handed over to the National Archives the morning after they were discovered.
The news comes as the Justice Department is investigating former President Donald Trump's handling of highly sensitive material, after FBI agents seized thousands of documents - about 100 of which were marked as classified - during a search of his Florida resort last August.
Trump and his supporters were trying to draw parallels on Tuesday between the two cases...
But Gregg Sofer, a partner with Husch Blackwell, says there are notable differences.
"Obviously, the more highly classified something is, the more serious potentially the damage is to the United States. From an evidentiary standpoint, it makes a big difference. It's easier to say ten documents were accidentally interspersed with non-classified information than it is to say 15 boxes were."
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"Generally speaking, for me and my practice and the clients that I represent, it often makes a big difference when something's voluntarily disclosed to the government versus the government having to go out there and dig it up themselves."
An important distinction, says Paul Charlton, a former U.S. Attorney in Arizona:
"The allegations as it relates to President Trump are much different. When the National Archives asked for President Trump and his team to return those documents, they returned some documents, but not all documents. A grand jury subpoena was issued. Some, but not all documents were returned. Search warrant was issued. And sure enough, a number of other remaining top secret documents were still in former President Trump's possession. That is a significant difference in the way in which these classified documents were reported both by President Trump and by President Biden. "
In a Monday statement, the special counsel to the president said the classified material was identified by personal attorneys for Biden on Nov. 2, days before the midterm elections.
It remains unclear what information the documents contained or their level of classification.
The Justice Department, the National Archives and the think tank did not respond to a request for comment.
