WASHINGTON: President Joe Biden was interviewed as part of an investigation into his handling of classified documents by Special Counsel Robert Hur, the White House counsel's office said on Monday.

The Democratic president's interview was voluntary and conducted over two days, concluding on Monday, said Ian Sams, a spokesperson for the White House counsel's office.

U.S. Attorney General Merrick Garland in January named Hur as special counsel to look into the improper storage of classified documents at Biden's Wilmington, Delaware, home and at a Washington think tank office set up for Biden after his 2009-2017 vice presidency.

Such special counsels are appointed to handle sensitive cases - usually involving political figures or allegations of serious wrongdoing by the government - and have more independence than is usually afforded to federal investigators to decide whether to bring charges.

Hur's interviews with Biden suggest the investigation, which has proceeded quietly, may have advanced to its final stages. Biden has said he was surprised by the discovery of classified materials and expected it would eventually be deemed inconsequential. His team said they have cooperated with the investigation.

"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 in a statement.

The White House declined to comment further, referring questions to the Justice Department, which did not immediately respond to a request for comment.

A wider-ranging inquiry into handling of classified documents has been directed at Biden's main Republican potential rival for the 2024 elections, Republican former President Donald Trump.

Trump was criminally indicted in that case, one of several legal cases against the former president, with the federal documents case in Miami set to go to trial in May. Trump has denied wrongdoing.