Special counsel Robert Mueller revealed in a court filing that former top Trump campaign aide Rick Gates had contacts with a former agent of Russia's intelligence service as late as October 2016 – weeks before the election that propelled Donald Trump into the White House.

The communications were disclosed in a sentencing memorandum filed Tuesday night against Alex van der Zwaan, a Dutch lawyer who pleaded guilty to lying to the FBI.

Mueller said Gates and van der Zwaan had numerous calls between September and October of 2016 with a "close business colleague" of Gates and Trump's campaign chief, Paul Manafort.

The colleague, identified in the memo only as "Person A," is alleged to have worked with Gates and Manafort "in connection with their Ukraine lobbying work." Person A is a Ukrainian foreign national who lived in Kiev and Moscow until mid-August 2016, the memo said.

The special counsel alleges that Gates told van der Zwaan of Person A's status as a former officer of Russia's military intelligence wing, which is known as GRU.

The special counsel is investigating Russian meddling in the 2016 election, as well as potential collaboration between the Trump campaign and the Kremlin. Trump has repeatedly denied colluding with the Russians. Gates stayed with the Trump campaign after Manafort was fired in summer 2016, and he served on Trump's inaugural committee. He is also a co-founder of the pro-Trump nonprofit advocacy group America First Policies.