WASHINGTON (AP) — 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, 2001 terrorist 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.

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.

Longtime aides say they believe Biden has been keeping personal diaries for decades, though the only public glimpse of them so far has come in Biden's book "Promise Me, Dad," which chronicled the then-vice president's heartache and grief over his son Beau's fatal cancer diagnosis.

In the book, Biden quotes passages written in his diary about Beau's condition and death that were written on Air Force Two, in the vice president's residence at the Naval Observatory in Washington, and at his Wilmington home, as well as one jotted down as he weighed whether to run for president in 2016. In the book, Biden describes taking the notes as he navigated being a supportive parent for an ailing family member and largely maintaining his official schedule of meetings and calls.

He details how he had a secure phone installed at MD Anderson Cancer Center in Houston so he could work while he was there with his son as Beau underwent treatment. But he also wrote about his debate over whether he'd run for office in 2016:

"'A lot happening,' I wrote in my diary when I finally got some downtime in Wilmington the next weekend. 'Need to be careful it doesn't get away from me. I need to slow down, ramp down my schedule.'"

It's unknown whether handwritten notes may have been turned over to the Department of Justice by former Vice President Mike Pence or whether any of former President Donald Trump's writings from his time in office was found during the FBI's search of his Florida estate last year.

It was also unclear whether recent former presidents and vice presidents would make any of their personal notes written during their time in office available for review to determine whether they contained any potential federal records or information that should be classified.

There's a precedent in keeping personal records personal: Access to Ronald Reagan's personal diaries was sought after he left office by his former national security adviser John Poindexter as he faced trial for his role in the Iran-Contra affair. A federal judge accepted Reagan's invocation of executive privilege to shield the diaries from disclosure.

Reagan frequently wrote about the substance of his official meetings — including details on classified sessions — and impressions of world leaders, often commingled with mundane details about his life like his dinner companions and personal calls. But it wasn't until after Reagan's death and with the consent of his widow, Nancy Reagan, that they were published.

There have been multiple cases in recent years of high-level officials mishandling notes about classified operations. Former CIA Director David Petraeus was prosecuted for his handling of eight notebooks of classified and unclassified notes he collected during his time leading U.S. and allied forces in Afghanistan. According to a plea agreement, Petraeus kept the notebooks in his private possession and allowed his biographer, with whom he was having an affair, to review them.

He pleaded guilty in 2015 to one count of unauthorized removal and retention of classified material and received probation.

Former Secretary of State Hillary Clinton was found by the FBI to have discussed classified material in emails kept on her private server. Some of those emails had classified information at the time they were sent, while others were subsequently classified during the FBI's investigation of her use of the server.

Then-FBI Director James Comey recommended against charging Clinton in 2016 because he said there was not clear evidence Clinton or her subordinates intended to violate laws about classified information.

Biden's lawyers were closing up his office at the Penn Biden Center think tank last November when they came across classified documents in a locked closet. The records were turned over to the Justice Department. But after Biden's lawyers searched his Wilmington home and found additional classified items, Attorney General Merrick Garland appointed a special counsel to investigate. Biden has said he was surprised the documents were there, and has cooperated with investigators, including voluntarily consenting to the FBI searches.

When FBI agents searched Biden's Wilmington home last month, they "also took for further review personally handwritten notes from the vice-presidential years," according to his lawyer, Bob Bauer. When the FBI searched Biden's Rehoboth Beach home on Wednesday, they took "some materials and handwritten notes that appear to relate to his time as Vice President" but found no other classified documents, according to Bauer.

The White House has refused to comment on what was in Biden's notes, other than to say some of the writing pertained to his time as vice president.

"I think that they want to make sure that the Justice Department has access to the information that they need to sift through materials as a part of this ongoing investigation," White House spokesman Ian Sams said Wednesday. "And so I'm not going to characterize too much of the underlying contents."