WASHINGTON (AP) — President Joe Biden has been interviewed as part of an independent investigation into his handling of classified documents, the White House said late Monday. It’s a possible sign that the investigation is nearing its end.

Special counsel Robert Hur is investigating the improper retention of classified documents from Biden’s time as a U.S. senator and as vice president that were found at his Delaware home, as well as at a private office that he used in between his service in the Obama administration and becoming president.

Biden has said he did not know the documents were there.

Ian Sams, a spokesperson for the White House counsel’s office, said in a statement that the interview was voluntary and conducted at the White House on Sunday and Monday.

Hur’s office likely asked to interview Biden sometime after Aug. 25.

Asked at that time if he planned to sit for an interview with the special counsel, Biden replied, “There’s no such request and no such interest.”

The interview could signal that the special counsel investigation is nearing its conclusion.

In 2016, then-FBI Director James Comey announced his recommendation against criminal charges for Hillary Clinton over her handling of classified information just three days after agents interviewed her at FBI headquarters.

Investigators with Hur’s office have already cast a broad net in the probe, interviewing a wide range of witnesses about their knowledge of how classified documents were handled.

In his statement, Sams reiterated that Biden and the White House were cooperating. He referred any questions to the Justice Department.

“As we have said from the beginning, the President and the White House are cooperating with this investigation, and as it has been appropriate, we have provided relevant updates publicly, being as transparent as we can consistent with protecting and preserving the integrity of the investigation,” Sams said. “We would refer other questions to the Justice Department at this time.”

Attorney General Merrick Garland named Hur in January 2023 to handle the politically sensitive investigation in an attempt to avoid conflicts of interest.

The investigation is separate from special counsel Jack Smith’s probe into the handling of classified documents by former President Donald Trump after he left the White House. Smith’s team has charged Trump with illegally retaining top secret records at his Mar-a-Lago home in Florida and then obstructing government efforts to get them back. No evidence has emerged to suggest that Biden engaged in comparable conduct or willfully held onto records he wasn’t supposed to have.

It is hardly unprecedented for sitting presidents to be interviewed in criminal investigations.

President George W. Bush sat for a 70-minute interview as part of an investigation into the leak of the identify of a CIA operative. President Bill Clinton in 1998 underwent more than four hours of questioning from independent counsel Kenneth Starr before a federal grand jury.

Special counsel Robert Mueller’s team negotiated with lawyers for then-President Donald Trump for an interview but Trump never sat for one. His lawyers instead submitted answers to written questions.