"They clearly are tying up loose ends," a lawyer said to be in contact with the special counsel's office told the outlet.
NBC News also cited "government officials and others familiar with the situation."
"The sources either did not know or would not say" whether Mueller had reached a conclusion on whether the campaign colluded with Russian spy agencies during the 2016 presidential election. Mueller is, however, expected to answer the question of whether Trump obstructed justice by firing FBI Director James Comey in the mid-February report.
Mueller took control of the FBI's Russia probe in May 2017. It was originally started in July 2016 under the direction of Comey and fired FBI Deputy Assistant Director Peter Strzok.
So far, Mueller has secured indictments against 33 people, many of whom — Russian nationals in particular — are expected to never face trial as the US and Russia don't have an extradition treaty. Mueller has also received guilty pleas from three high-ranking members of the campaign: former manager Paul Manafort, former White House National Security Advisor Michael Flynn, and former longtime attorney for Trump Michael Cohen. George Papadopoulos, a low-ranking foreign policy advisor, has also pleaded guilty.
However, Mueller has failed so far in securing any indictments under the primary mandate of his probe: investigating "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump."
The convictions so far have fallen under the secondary objective of Mueller's job: investigating "any matters that arose or may arise directly from the [aforementioned] investigation." Those convictions have largely been on charges of lying to authorities or financial crimes committed prior to the 2016 election.