Former Trump deputy campaign chairman Rick Gates and the unidentified person were in touch in September and October 2016, a fact that prosecutors say is "pertinent to the investigation," according to a new court filing, Press TV reported.
The filing released on Tuesday night is connected to the sentencing of Dutch lawyer Alex van der Zwaan, who pleaded guilty in February to lying to the special counsel about his interactions with Gates.
In an opposing 30-page memorandum also filed Tuesday evening, van der Zwaan implored for leniency and asked the judge to let him pay an "appropriate fine" instead of sending him to jail next week.
The Dutch lawyer will be the first defendant in Mueller's probe to face sentencing on April 3.
"(H)is world has collapsed as a result of his decision to lie to law enforcement," says the court filing from the Dutch lawyer.
Mueller, a former FBI director, has been running a high-profile investigation into allegations that Trump won the 2016 US presidential election against Democratic rival Hillary Clinton only because Moscow had rigged the election in his favor.
US intelligence agencies claim Russia-linked hackers provided WikiLeaks with damaging information -- in the form of thousands of hacked emails -- about Clinton to skew the 2016 presidential election in favor of Trump.
Trump has repeatedly denied allegations that his campaign colluded with Russians and has condemned the investigations. Russian President Vladimir Putin has also denied the allegations.