An associate of a Russian co-defendant of ex-Trump campaign chief Paul Manafort pleaded guilty Friday to failing to register as an agent for foreign interests, and has agreed to cooperate with prosecutors, including special counsel Robert Mueller.

The case against W. Samuel Patten, 47, of Washington, D.C., relates to consulting and lobbying work that he and an unidentified Russian national performed for a Ukraine political party and its members, who included a "prominent Ukraine oligarch." Manafort had advocated for the creation of that party, Opposition Bloc, after Manafort's client, ex-Ukraine President Viktor Yanukovych, was removed from office and fled to Russia.

Patten concealed the fact that he had caused $50,000 from that unidentified oligarch to be used to buy four tickets for the oligarch to attend the presidential inauguration of Donald Trump with Patten in January 2017, according to court records. The inauguration committee is barred from accepting money from foreign nationals.

Patten's case was referred by Mueller to the U.S. Attorney's Office for the District of Columbia and the Justice Department's national security division.

Mueller is prosecuting Manafort and also is investigating Russian interference in the 2016 election that put Trump in the White House.

The charge against Patten, violating the Foreign Agents Registration Act, mirrors one pending against Manafort, carries a maxmium possible sentence of five years in prison.

A spokesman for prosecutors declined to comment when asked if Patten is now expected to testify against at Manafort's trial next month in the same Washington courtroom where Patten pleaded guilty Friday. Patten's lawyer, Stuart Sears, declined to comment.

Patten was freed without bond by Judge Amy Berman Jackson at his court appearance. The judge ordered Patten to continue participating in mental health services, to surrender his passport, to not consume alcohol and to not travel outside of the Washington, D.C., metropolitan area without permission from Jackson.

Patten has ties to Konstantin Kilimnik, the suspected Russian intelligence agent who in June was charged with Manafort for trying to tamper with potential witnesses against Manafort in pending criminal cases. Kilimnik had been an aide to Manafort.