Special counsel Robert Hur’s interview of President Joe Biden is a sign that the classified documents investigation is nearing conclusion after casting a wide net that included dozens of witnesses during the ten-month long probe, multiple sources told CNN.

The White House announced this week that Biden was questioned by Hur and his team over two days in a voluntary interview that CNN has reported was scheduled weeks earlier. While the White House has declined to discuss details of the questioning, including whether Biden invoked executive privilege, the interview is the first public development in months.

One source told CNN that investigators have indicated they hope to wrap by the end of the year. As of now, it’s unclear if the probe will result in charges being filed, but sources familiar with the investigators’ line of questioning said they got the impression that’s unlikely, and there has been no discernible grand jury activity.

The Justice Department has said that Hur will produce a final report explaining his findings from the investigation, a standard part of a special counsel’s work.

“The breadth and depth of Hur’s work suggests that he is going to compile a detailed report to explain exactly how he conducted this investigation,” one source familiar with the investigation told CNN.

Hur was appointed in January to investigate after classified documents were found at Biden’s former office at the Penn Biden Center in Washington, DC, and at his Wilmington, Delaware, home.

Compared to special counsel Jack Smith’s investigation into classified materials found at former President Donald Trump’s Mar-a-Lago residence, including the indictment handed down in June, Hur’s probe into Biden has continued to operate quietly behind the scenes.

Still, the protracted length and exhaustive nature of the investigation has frustrated top Biden aides who expected it to wrap up months ago given the relatively small number of classified documents involved, according to a person familiar with the White House’s thinking.

That person said some Biden aides believe Attorney General Merrick Garland was overly cautious in selecting Hur, an appointee to two top Justice Department roles during the Trump administration, to ensure the investigation was politically unassailable.

Investigators working for Hur have interviewed a broad spectrum of witnesses — from longtime advisor and current counselor Steve Ricchetti, to former White House legal and communications aides, to a former low-level aide who helped pack up the vice president’s residence at the end of the Obama administration, sources familiar with the matter tell CNN.

Hur’s team also has reached out to people who worked in the Senate during the time Biden served in that chamber, sources said. That’s because some of the documents with classified markings date back to Biden’s time in the Senate, according to a statement from Biden’s personal attorney.

As part of the investigation, Hur has sought to examine the handling of classified documents during Biden’s time in the Senate, a period before many of the strict procedures now used for handling classified documents, according to sources.

That has caused Hur to confront the delicate issues of the Senate’s constitutional speech or debate protections, which limit the Justice Department’s ability to interview Senate staff without coordination with Senate lawyers, people briefed on the matter said.

It’s unclear whether and how the Senate and Justice Department’s discussions over Senate-related interviews have been resolved. A special counsel spokesman declined to comment on the idea of no charges or on any discussions with the Senate.

One person familiar with the investigation described members of Hur’s team as being professional but tedious in the level of detail they have sought in witness interviews. Investigators have asked about where staffers sat in the office, where they stored briefing books, and how they operated an office safe.

Another person described a lengthy interview with FBI agents and lawyers focused on understanding everything surrounding specific documents. Investigators appeared to be following a process that identified meetings connected to specific classified documents or notes recovered from Biden properties, the person familiar with the interview said. Everyone who attended a meeting or briefing connected the document is being interviewed, the person said.

Investigators appear to be trying to establish a chain of custody for the documents and the circumstances surrounding them to discern how the classified documents ended up in Biden’s office and home.

Another source said: “The central question in this case is: Did the vice president of the United States intentionally take classified documents for personal use?”

The challenge for investigators is how they assess culpability and the circumstances surrounding how the documents got to the Penn Biden Center and the president’s house in Delaware, the source said.

A lawyer for one witness also described Hur’s process as being slow and methodical. Investigators interviewed this lawyer’s client earlier this year, but recently came back and asked his witness for additional documents.

“They are certainly being sufficiently thorough, and there is a temptation to think they are doing some things twice,” the lawyer said.

White House press secretary Karine Jean-Pierre declined to say Tuesday if the president answered all questions posed to him or invoked executive privilege during the interview with Hur. Jean-Pierre also wouldn’t say if the Biden administration requested that the interview be postponed following Hamas’ attack on Israel over the weekend.

“He’s been very much focused on the issues of the – you know – horrific events that we have seen in Israel,” she said. “As president, he has to do multiple things at once, and that’s what you saw him do this weekend.”