A month before the 2016 presidential election, a campaign aide to Donald Trump directly communicated with a man he knew was a onetime Russian spy, according to a court filing by special counsel Robert Mueller.

Rick Gates, the campaign aide, told an attorney for a U.S. law firm that a Russian national they worked with in Ukraine was a former military intelligence officer, according to the filing late Tuesday in Washington federal court.

Both Gates and the lawyer, Alex van der Zwaan, have pleaded guilty as part of Mueller's investigation of Russian meddling in the election. Gates spent a decade working for Paul Manafort, the indicted former Trump campaign chairman, as a political consultant in Ukraine.