A business associate of a key figure in the investigation into former Trump campaign chairman Paul Manafort pleaded guilty Friday to failing to register as a foreign agent for a Ukrainian political party.

W. Samuel Patten, 47, also admitted to soliciting a U.S. citizen to act as a straw purchaser to buy tickets to the inauguration of President Donald Trump for a Ukrainian oligarch, circumventing a law restricting such purchases by foreigners.

The plea agreement, which includes cooperating with special counsel Robert Mueller's office, raises the prospect Patten will be called to testify against Manafort, who faces a second trial in Washington next month, or his business associate Konstantin Kilimnik, who was indicted for witness tampering in the same case.

The Patten case was referred by Mueller's team to the United States attorney's office in Washington, said Bill Miller, a spokesman for the office. Andrew Weissmann, one of the lead Mueller team attorneys in the Manafort prosecution, was seen at court Friday ahead of Patten's appearance

Patten was a business associate of Kilimnik, a man U.S. authorities have said has ties to Russian intelligence.

Kilimnik worked closely with Manafort, who was found guilty this month of eight financial counts. Kilimnik also is a co-defendant in a pending case against Manafort in Washington, brought by Mueller's team, that accuses them both of witness tampering.

Court papers don't refer to Kilimnik by name, but say Patten worked with a Russian national on lobbying and political consulting services.

Prosecutors say Patten, who formed a consulting company with a person identified only as "Foreigner A," worked to set up meetings with members of Congress and also drafted talking points for Capitol Hill meetings. They say Patten was working on behalf of a wealthy Ukrainian, who was a member of the Ukrainian political party known as the Opposition Bloc. The businessman is not named and referred to only as "Foreigner B."

The goal, prosecutors say, was to influence U.S. policy, but they say Patten never filed under the Foreign Agents Registration Act or disclosed that he was representing the foreign businessman and the Opposition Bloc. The law is aimed at promoting transparency about lobbying efforts in the United States.

U.S. District Judge Amy Berman Jackson told Patten that she couldn't provide any estimate of Patten's potential sentence because U.S. sentencing guidelines don't have a section for violations of the Foreign Agents Registration Act. Prosecutions of the offense are rare, but in recent years the Justice Department's national security division has taken a tougher stance on enforcement of the law.

Patten's plea agreement requires him to cooperate with the government. He was released on his own recognizance Friday without a sentencing date.