Antony Blinken
Antony BlinkenREUTERS/Sarah Silbiger

The federal investigation into US President Joe Biden's handling of classified documents prior to becoming President has grown into a sprawling examination of Obama-era security protocols and internal White House processes, with investigators so far interviewing scores of witnesses, including Secretary of State Antony Blinken, sources familiar with the investigation told ABC News on Tuesday.

Federal prosecutors and FBI agents from special counsel Robert Hur's office have been interviewing witnesses for nearly nine months, including high-level advisers, executive assistants and at least one White House attorney, the report said.

Several sources estimated that as many as 100 witnesses have already been interviewed, with interviews conducted as recently as last week and some witnesses asked to return for follow-up interviews.

In December of last year, a White House lawyer said that a “small number of documents” with classified markings that are tied to Biden and appear to be from the Obama administration are under review by the Justice Department and National Archives.

Two days later, it was reported that aides to Biden discovered at least one additional batch of classified documents in a location separate from the Washington office he used after leaving the Obama administration.

The White House said in mid-January it had discovered five additional pages of classified documents at Biden's home, the same day a special counsel was appointed to review the matter.

At the start of February, the FBI searched Biden's beach house in Rehoboth Beach, Delaware, but did not find any new classified documents.

In January, shortly after news first surfaced that the classified documents had been found, Blinken, a long-time aide to Biden, said he and Biden were both "surprised to learn that there were any government records taken."

Blinken also told reporters he "had no knowledge of it at the time" but "would cooperate fully" with the Justice Department's review.

ABC News noted that it is unclear what Blinken told Hur's team in his voluntary interview with them.

Spokespersons for Hur, the State Department, Biden's personal attorney, and the White House declined to comment. An attorney representing Blinken also declined to comment.