Mueller was appointed to investigate any links between Trump associates and Russian Federation, as well as other matters that arise out of his investigation.
Dowd, who resigned as Trump's lawyer last week, also floated the offer of a pardon last summer to lawyers for former national security adviser Michael Flynn, according to a report Wednesday by the New York Times.
Speaking at the daily briefing, spokeswoman Sarah Huckabee Sanders read a statement White House lawyer Ty Cobb provided to the Times, in which he said, "I have only been asked about pardons by the press and have routinely responded on the record that no pardons are under discussion or under consideration at the White House".
Dowd denied to the Times that he discussed pardons with the lawyers for Flynn and Manafort.
What matters? That van der Zwaan had had communications, in the heat of the presidential election, with Rick Gates, a longtime business partner of Paul Manafort and Donald Trump's campaign deputy who is now facing criminal charges of his own.
"Federal Bureau of Investigation Special Agents assisting the Special Counsel's Office assess that Person A has ties to a Russian intelligence service and had such ties in 2016", Mueller said in the filing.More news: South Korea steel shares rise on United States steel tariff exemption
Dowd's reported message to the defendants "puts us, yet again, in uncharted waters", he added.
A grand jury had already been convened to look into evidence against Flynn by the time Dowd spoke with Kelner, the Times reports, while the conversation with Brown took place prior to an indictment handed down in October charging Manafort for various financial crimes.
Three sources say that Mueller's team is now looking at the possibility that Dowd made pardon offers to both former Trump advisers past year because the president's team was concerned about what they might say if they struck a deal with the special counsel.
The other new piece here is that they say Gates described Person A (again, apparently Kilimnik) as "a former Russian Intelligence Officer with GRU".
The White House and Trump's lawyers have regularly argued that the president is not considering pardons. And Sanders had a reminder of her own for reporters: "The president has the authority to pardon individuals".
"A president pardoning defendants if they don't plead guilty (and cooperate is the unspoken word here) would be a new use of the pardon power", he said.
"I hope they're right", he said.