The federal investigation into 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.

Federal prosecutors and FBI agents from special counsel Robert Hur's office have been interviewing witnesses for nearly nine months, targeting an expansive constellation of former aides -- from high-level advisers to executive assistants and at least one White House attorney. 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.

Sources who were present for some of the interviews, including witnesses, told ABC News that authorities had apparently uncovered instances of carelessness from Biden's vice presidency, but that -- based on what was said in the interviews -- it seemed to them that the improper removal of classified documents from Biden's office when he left the White House in 2017 was more likely a mistake than a criminal act.

Nevertheless, the special counsel has reached no final determinations and the investigation is ongoing, ABC News was told.

In January, shortly after news first surfaced that classified documents had been found at a personal office used by Biden after his vice presidency, Blinken, a long-time aide to Biden, said he and Biden were both "surprised to learn that there were any government records taken." It's unclear what Blinken told Hur's team in his voluntary interview with them.

For a high-stakes special counsel investigation into a sitting president, Hur has operated largely under the radar since his appointment in January -- avoiding the attention and media scrutiny of special counsel Jack Smith's probes into former President Donald Trump.

But ABC News learned from sources that Hur's team has cast a wide net, gathering documents dating back to the early days of the Obama administration and drilling into questions about the task of securely updating the vice president on highly sensitive matters.

Investigators have shown witnesses email chains dating back to at least 2010 and asked for context about those exchanges, sources said. Witnesses have also been pressed about the use of cabinets and safes, sources said.

It has been publicly reported that investigators searched for documents dating back to Biden's tenure in the Senate.

Sources said investigators are asking witnesses, especially former military aides, granular questions about internal procedures for handling classified materials, apparently seeking to understand the minutiae of how the vice president obtained, consumed, and discarded classified briefing materials.

Of particular interest to investigators, according to multiple sources, was any context surrounding Biden's tendencies for notetaking and document retention, including where he stored documents, briefing books, notes, and notecards. Prosecutors also asked the witnesses about an iPad and cell phone Biden kept for personal use, and whether they were aware if he ever handled classified materials on those devices, the sources said.

Investigators also asked witnesses about how Biden's closest aides handled classified records, according to sources. Michelle Smith, a former executive assistant to Biden who is now deceased, has been brought up in some interviews. ABC News previously reported that Kathy Chung, Smith's successor as executive assistant, met with investigators earlier this year.

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

Hur has vowed to conduct a "fair, impartial, and dispassionate" investigation, following the facts "thoroughly" and "without fear or favor."

A series of revelations precipitated Hur's ascent to special counsel. In late 2022, the White House told the National Archives that documents bearing classification markings had been discovered at the Penn Biden Center in Washington, D.C. -- the location of Biden's private office after his term as vice president expired in early 2017.

From 2017 to 2019, Blinken was the managing director of the Penn Biden Center, and before that he served as Biden's national security adviser during President Barack Obama's first term, with their association going back more than two decades, beginning when Biden was still a U.S. senator.

After the classified documents were found at the Penn Biden Center late last year, Blinken told reporters he "had no knowledge of it at the time" but "would cooperate fully" with the Justice Department's review. The White House similarly said that it would cooperate.

Biden's personal attorney later informed investigators that additional classified records were identified in the garage of Biden's Wilmington, Delaware, home -- a development that marked a tipping point in the Justice Department's decision to appoint a special counsel to investigate further, sources told ABC News in January.

In all, about 25 documents marked classified were found in locations associated with Biden.

Biden has sought to downplay his legal exposure. One week after Hur's appointment, in response to reporters' questions about why he did not reveal the documents before November's midterm elections, Biden replied that "we found a handful of documents" that had been "filed in the wrong place" and that he was cooperating with the National Archives and the Justice Department.

"I think you're going to find there's nothing there," he said.

Meanwhile, special counsel Smith's team has aggressively pursued former President Trump and two aides in his investigation into Trump's handling of classified records after leaving office. In their indictment against Trump, prosecutors allege a months-long conspiracy to block government efforts to retrieve classified documents from his Mar-a-Lago estate in the summer of 2020, including by allegedly hiding those documents from the FBI and Trump's own attorney.

According to the indictment, when the FBI then searched Mar-a-Lago in August of last year, agents found more than 100 documents marked classified that Trump allegedly failed to turn over in defiance of a federal grand jury subpoena.

In Biden's case, all of the classified documents found in locations associated with Biden were voluntarily provided to the government, Biden's lawyers said at the time.

"We are confident that a thorough review will show that these documents were inadvertently misplaced," an attorney for Biden said in a statement after Hur's appointment.

Trump and his aides have denied the allegations against them and pleaded not guilty.

ABC News' Pierre Thomas contributed to this report.