These investigators have called the White House to produce documents about Michael Flynn, the former national security adviser to president Donald Trump, according to the daily, citing officials close to the investigation.
Although the request is not a subpoena, this is the first time the White House has been directly asked to transfer documents to the Mueller's investigation team.
The newspaper said the investigators had questioned witnesses about whether Flynn was secretly paid by the Turkish government during the campaign.
They are said to have questioned witnesses about his dealings with Turkish-American lobbyist Ekim Alptekin, who paid his consulting firm Flynn Intel Group $530,000 during the United States election a year ago.
Reportedly, Mueller and his team are now investigating not only whether Flynn failed to report his links to a foreign government as required by law or if he had illicit ties with Russian elements, but all of Flynn's business connections throughout the campaign.
Despite his mandate to look into allegations of President Donald Trump's collusion with Russian Federation, the counsel has lately been focusing on possible financial crimes of the president's team.
The White House and Flynn's lawyer, Robert Kelner, did not immediately respond to requests from Reuters for comment. If that money actually came from the Turkish government itself, Flynn could face a felony charge for not declaring himself a foreign agent or other charges for trying to hide the source of the money.
Prosecutors have also asked during interviews about Flynn's speaking engagements for Russian companies, for which he was paid more than $65,000 in 2015, and about his company's clients - including work it may have done with the Japanese government.