WASHINGTON • President Joe Biden is a man who writes down his thoughts. And some of those handwritten musings over his decades of public service are now a part of a special counsel’s investigation into the handling of classified documents.

It isn’t clear yet what the investigators are looking for by taking custody of notes from his time as vice president and his decades in the Senate that were found in his Delaware homes in Rehoboth Beach and Wilmington.

Biden’s attorneys did not say whether the notes were considered to be classified, only that they were removed. But over his 36 years in the Senate and eight as vice president, Biden had a front-row seat to a lot of highly sensitive moments in U.S. history, including the Sept. 11 attacks, the 2011 death of Osama bin Laden and unfolding political turmoil in Ukraine.

The special counsel is working to determine how classified information from Biden’s time as senator and vice president came to wind up in his home and former office — and whether any mishandling involved criminal intent or was unintentional. But they’ll also have to determine whether the notes they took are considered personal and therefore belong to Biden, and would then likely be returned to him.

Some of the documents held by Trump also had handwritten notes, according to the FBI.

In seeking permission to search Trump’s Mar-a-lago estate in August, an FBI agent wrote in an affidavit that some of the documents returned to the National Archives last January contained what appeared to be Trump’s handwriting.

The affidavit does not say whether agents believed those notes to discuss classified material.

Under the Presidential Records Act, records of a presidential administration generally belong with the National Archives, especially classified items. There are some exceptions, including when records are determined to be purely personal.

But even a handwritten note can be considered classified if someone is recording observations related to a classified document or briefing. Such notes can be deemed classified even if not marked as such.

Larry Pfeiffer, a former senior director of the White House situation room and chief of staff to retired CIA Director Michael Hayden, said that when he took notes during secret or top-secret meetings, he would mark each page by specific levels of classification.

“It’s pretty clear in those meetings when they’re hearing classified information,” he said.

When Pfeiffer left the CIA, he submitted his notebooks to the agency archives.

The leaders of the Senate Intelligence Committee sent a third letter Thursday urging Garland and Haines to allow the panel to view the papers in secret and be briefed on their potential risk to national security.

Sens. Mark Warner of Virginia and Marco Rubio of Florida wrote that without access to the documents, “we cannot effectively oversee the efforts of the Intelligence Community to address potential risks to national security arising from the mishandling of this classified information.”