Issued within the past month, the directive marks the special counsel's first records request to the Justice Department, and it means Mueller is now demanding documents from the department overseeing his investigation.
Mueller's investigators now seek not only communications among Justice Department staffers but also any of their communications with White House officials, the source said. Before this request, investigators asked former senior Justice Department officials for information from their time at the department, ABC News was told.
The latest move suggests the special counsel is still digging into, among other matters, whether Trump or any other administration official improperly tried to influence an ongoing investigation.
Last month Sessions told lawmakers he would cooperate with any requests from Mueller and is willing to meet with him.
"I want him to complete his investigation professionally," Sessions told the Senate Judiciary Committee.
Shortly before firing Comey, Trump secretly drafted a memo laying out his reasons for wanting the FBI chief ousted. The New York Times described it as an "angry, meandering" missive.
The draft memo was never publicly released, but a copy was shared with Rosenstein, who had taken command of the Russia-related probe, according to the Times.
To publicly bolster Trump's decision on Comey, the White House released two memos written separately by Sessions and Rosenstein, with both faulting Comey for his handling of the FBI's probe into Hillary Clinton's use of a private email server when she was secretary of state.
During a House hearing in June, Rosenstein refused to say whether he consulted with the White House before Comey's firing or whether anyone asked him to write his memo, insisting such questions "may well be within the scope of the special counsel's investigation."
Rosenstein maintains final supervision over the case, even though he was interviewed by Mueller's team about his role in Comey's firing.
Meanwhile, Trump has taken aim at Sessions for the recusal, launching such biting personal attacks months ago that it appeared that Sessions would not last the summer as attorney general.
At one point, Trump told reporters he wouldn't have nominated Sessions to run the Justice Department had he known Sessions would give up oversight of the investigation.
In July, Trump posted a tweet demanding to know why "our beleaguered" attorney general wasn't "looking into Crooked Hillarys crimes & Russia relations."
In announcing his recusal four months earlier, Sessions said he and "senior career department officials" spent "several weeks" discussing whether his role as top foreign policy adviser to Trump's presidential campaign last year meant his "impartiality might reasonably be questioned."
His work leading the campaign's foreign policy team has left Sessions on the defensive in other ways.
Last week, Senate and House Democrats hammered Sessions for previously telling Congress — under oath — that no Trump campaign associates ever communicated with Russian operatives or intermediaries.
Some Democrats accused Sessions of lying to lawmakers, though he has vehemently denied the charge, citing a memory lapse due in part by the "chaos" of the campaign.
During a House hearing Wednesday, Sessions said he now remembers dismissing Papadopoulos' proposal during the meeting last year.
Papadopoulos has pleaded guilty to charges of lying to the FBI about his contacts with Russian nationals.
Meanwhile, other Trump associates, such as former national security adviser Mike Flynn, are still in Mueller's crosshairs.
Flynn was fired in February after then–acting Attorney General Sally Yates informed White House officials that Flynn lied to them about his contacts with Russian officials.
A spokesman for Mueller declined to comment for this article. A spokeswoman for the Justice Department also declined to comment.