Rick Gates, the former Donald Trump campaign adviser who pled guilty in February to conspiracy against the United States and making false statements, was “having substantive conversations” with someone he knew was a former Russian Intelligence Officer in 2016, according to a new filing by special counsel Robert Mueller.

The 30-page sentencing memorandum filed by the special counsel alleges that Gates was communicating with “Person A,” who is described as “a former Russian Intelligence Officer with the GRU”—the foreign military intelligence agency of the Russian Federation. According to the document, Gates was aware of “Person A’s” connection to Russian intelligence services.

The memorandum was filed with the United States District Court for the District of Columbia in the case U.S. v. Alex van der Zwaan.

Van der Zwaan—the son-in-law of Russian oligarch German Khan, who is suing BuzzFeed News over its publication of the Trump-Russia dossier—pleaded guilty in February to lying to the special counsel. According to the sentencing motion filed Tuesday, van der Zwaan’s lies “were material” to the special counsel’s investigation, and directly involved communications with Gates and “Person A,” a former GRU officer.

“That Gates and Person A were directly communicating in September and October 2016 was pertinent to the investigation,” the special counsel argued in its sentencing memo. ”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.”

“During his first interview with the Special Counsel’s Office, van der Zwaan admitted that he knew of that connection, stating that Gates told him Person A was a former Russian Intelligence Officer with the GRU,” the special counsel alleges

The special counsel’s filing also noted van der Zwaan “lied regarding topics at the center of a significant investigation into the criminal conduct of [former Trump campaign chairman] Paul J. Manafort, Jr., and Richard W. Gates III.”

According to the memorandum, both Manafort and Gates “had been indicted just days” before van der Zwaan lied to the special counsel—despite having been “expressly warned by the government that it is a crime to lie to the Special Counsel’s Office.”

Read the full memorandum here.