United States President Joe Biden was interviewed voluntarily as part of an investigation into the handling of classified documents found at his home and former private office, the White House said.

The 80-year-old president was interviewed by Special Counsel Robert Hur over two days concluding on Monday, said Ian Sams, a spokesperson for the White House counsel’s office.

The US attorney general appointed a special counsel in January to look into the handling of the secret files, which dated from Biden’s time as vice president under President Barack Obama and immediately afterwards.

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. Hur’s team previously carried out extensive interviews among Biden’s staff, ABC News said.

“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.

Former President Donald Trump, Biden’s probable Republican opponent in next year’s presidential election, separately faces trial over the alleged mishandling of classified documents after leaving office.

The special counsel in that case, Jack Smith, says Trump allegedly took classified documents to his Mar-A-Lago estate in Florida and refused to return them.

The case is set to go to trial in May. Trump has denied wrongdoing.