The preparations are the clearest sign to date that Mueller has almost completed his investigation of nearly two years.
The precise moment of the announcement is subject to change.
The scope and outlines of what Barr will send to Congress remain unclear. It is also difficult to know how long it will take for justice officials to prepare what will be submitted to legislators.
The Justice Department and the Special Council Office declined to comment.
Barr said that he wanted to be as "transparent" as possible with Congress and the public, "in accordance with the rules and the law".
Under the Special Councils Regulation, Mueller is required to submit a "confidential" report to the Attorney General upon completion of his work, but the by-law does not require it to be communicated to Congress or, by extension, to the public. And, as Barr has made clear, the Department of Justice generally refrains from disclosing "override" information about non-charged individuals.
As a result, one of Barr's most pressing issues in the coming weeks is to what extent Mueller's findings should be disclosed to Congress.
The bylaw requires Mueller to explain in his report all decisions to continue or not to proceed. Barr would also need to inform Congress if the Department of Justice prevented the team of legal advisers from continuing any investigative proceedings.
Speculation on the end of the probe has been generalized in Washington. NBC News recently reported that the investigation would be completed by mid-February.
Life after Mueller
Although the Mueller investigation may soon be over, prosecutions will still be conducted by other federal prosecutors.
In addition, Mueller dismissed some cases that do not enter the field of investigation conducted by Russia to other US lawyers. Some of these investigations have already been revealed, including the investigation in New York on the former lawyer of Trump's former lawyer, Michael Cohen. This investigation led to subsequent federal investigations in New York on the Trump Organization and the Trump Inaugural Committee. It is possible that the team of the special advocate has returned other cases that have not yet been revealed.
For the watchful observers of the Federal Court House and the Mueller team, small changes have been added in recent weeks.
On Monday, Tuesday and Wednesday of last week, the staff of the special advocates' office carried cartons and pulled out a cart full of files, which is unusual and could foreshadow a transfer of legal duties.
At the same time, the workload of Mueller prosecutors seems to be dwindling. Four of Mueller's 17 prosecutors ended their term with the bureau, most of whom returned to other roles in the Justice Department.
And the grand jury that Mueller's attorneys had the habit of making indictments of Trump's long-time confidant, Roger Stone, Paul Manafort, former chairman of the Trump campaign, and several Russians do not believe it's true. is apparently not meeting since Jan. 24, the day he approved the criminal charges against Stone.
Even with these signs of closure, DC's US Attorney's Office stepped in to work on cases that could last longer than Mueller advises.
This office joined some of Mueller's team records, including the case against Stone, a Russian propaganda plot in social media, as well as in the struggle of a foreign government-owned company against the Assignment of a grand jury.
Mueller and his attorneys always go to work – some even arriving on the last days of snow and presidents' day. However, prosecutors from the US Attorney's Office in DC and those at the Department of Justice who have previously worked on Mueller cases are also visiting them more often than ever before.
In a court case, Concord Management has been accused of supporting prosecutors for social media plot. They told a judge in January that there was still a "related case taking place before the grand jury".
In other cases, including Manafort, Mueller's team has made heavy redactions in recent court cases, including to protect ongoing investigations and those not yet charged with crimes. .
Laura Robinson Sam Fossum, CNN, Em Steck, contributed to this report.