The wide-ranging investigation of Russian meddling in the 2016 campaign took an unexpected diversion to Capitol Hill on Friday, when former Trump campaign aide Rick Gates pleaded guilty to lying to investigators about a meeting between his boss Paul Manafort and an unnamed member of Congress.

Gates admitted, according to court documents, that the 2013 meeting was part of a secret multimillion-dollar lobbying campaign for the Ukrainian government and its Russian-backed president, Viktor Yanukovych. Gates also pleaded guilty to conspiring with Manafort to hide the millions of dollars they were paid by the Ukrainian government.

Gates told a federal judge that he understood that his potential sentence depended on his cooperation with prosecutors on the charges: conspiracy to commit to defraud the United States and making false statements, CNN reported.

The plea is considered a key development. Gates is the third Trump associate known to be working with special counsel Robert S. Mueller III’s investigation. Gates was also a longtime associate and right-hand man to Manafort,the onetime chairman of Trump’s presidential campaign. Mueller is reportedly trying to pressure Manafort to cooperate as well.

Public filings show that the the March 19, 2013 meeting detailed in court documents coincided with a meeting between Manafort and Dana Rohrabacher, a Russia-friendly Republican congressman from California, Bloomberg reported Friday.

The meeting also included Vin Weber, a partner at Mercury Public Affairs, according to Bloomberg. Court documents indicated a lobbyist working for one of two companies Manafort had hired to help with his work on behalf of Ukraine was in the room.

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“As the congressman has acknowledged before, the meeting was a dinner with two longtime acquaintances — Manafort and Weber — from back in his White House and early congressional days,” Rohrabacher spokesman Ken Grubbs said in an email.

“The three reminisced and talked mostly about politics. The subject of Ukraine came up in passing. It is no secret that Manafort represented Viktor Yanukovych’s interests, but as chairman of the relevant European subcommittee, the congressman has listened to all points of view on Ukraine. We may only speculate that Manafort needed to report back to his client that Ukraine was discussed,” Grubbs said.

Gates told investigators on Feb. 1 that Manafort told him that nobody brought up Ukraine at the meeting. But Gates knew that was not true, partly because he and Manafort wrote a report for Ukrainian leadership about the “pertinent Ukraine discussions that Manafort had told him had taken place,” according to court documents.

Gates and Manafort lobbied “multiple members of Congress and their staffs,” about Ukrainian sanctions, the validity of Ukrainian elections and the propriety of Yanukovych imprisoning his presidential rival, according to court documents.

Reports late Friday said Mueller filed new charges against Manafort, accusing him of secretly paying former European politicians to lobby on behalf of Ukraine.

Gates and Manafort were charged in October with money laundering and failing to register foreign lobbying work with the U.S. government. Manafort has pleaded not guilty to all the charges against him.