WASHINGTON >> An American lobbyist who worked with Russian and Ukrainian oligarchs pleaded guilty today to failing to register as an agent of a foreign power and disclosed to prosecutors that he helped a Russian political operative and a Ukranian businessman illegally purchase four tickets to President Donald Trump’s inauguration.

Prosecutors disclosed that the inauguration tickets were worth $50,000 and were purchased with funds that flowed through a Cypriot bank account. Prosecutors did not name the foreigners involved. However, the tickets were purchased for Konstantin V. Kilimnik, a Russian political operative believed to have ties to a Russian intelligence agency, and a Ukranian oligarch.

The lobbyist, Sam Patten, also agreed to cooperate with prosecutors as part of his plea agreement. He could provide prosecutors insight into a range of activity and individuals relevant to the special counsel investigation, as well as connections between Trump, his associates and Russia.

Prosecutors said they would not bring additional charges against Patten for giving false statements to and obstructing the Senate Intelligence Committee. The committee, which questioned Patten on Jan. 18, is among the congressional panels that are conducting investigations into Russia’s election interference.

Foreigners are not allowed to contribute any money to the organization that runs presidential inaugurations. According to federal documents, Patten misled the Intelligence Committee several times and intentionally withheld documents that could have revealed the foreign purchase of the tickets.

Details disclosed in the plea agreement were the latest reminder of the far-reaching effects of the investigation by the special counsel, Robert Mueller, into Russia’s interference in the 2016 presidential election.

Trump has called the special counsel investigation a “witch hunt” and “rigged.” And allies of Trump have complained that Mueller’s mandate — to investigate Russia’s election interference as well as any related crimes that are unearthed in the process — is too broad. In January, Paul Manafort, Trump’s former campaign chairman, unsuccessfully sued the special counsel, asking the court to narrow Mueller’s scope.

Patten has worked in the same world as Manafort, who was recently convicted of financial fraud and is scheduled to go on trial in September over different charges.

As recently as this year, Patten worked as a foreign agent for the Opposition Bloc, a pro-Russia Ukrainian political party, according to a court document filed today. Manafort worked for the Opposition Bloc in 2014.

Manafort has also been charged with failing to register as a foreign agent for his work on behalf of pro-Russia Ukrainian interests, including the Opposition Bloc.

For decades, prosecutors have rarely brought charges for violating the Foreign Agents Registration Act, or FARA. But that changed, even before the appointment of the special counsel, as the Justice Department worked to meet the counterintelligence threats from foreign adversaries.

Mueller has pursued other investigations into lawyers and lobbyists who did work in Ukraine, and he has handed some of those cases off to federal prosecutors in other jurisdictions. A lawyer working for the special counsel was present in the courtroom today when Patten entered his guilty plea.

Patten’s lawyer declined to comment after today’s hearing. Patten faces up to five years in prison and a fine of up to $250,000. He was released after the hearing.

Patten’s work for the Opposition Bloc was done in conjunction with Kilimnik, who served as Manafort’s longtime deputy on the ground in Kiev. Mueller and his team have accused Kilimnik of having ties to Russian intelligence. Kilimnik was indicted in June on charges of obstruction of justice related to witness tampering in the foreign lobbying case against Manafort.

Kilimnik was not named in today’s charging papers, but is identified as “Foreigner A,” described as a Russian national who formed a company in the United States with Patten. That company, identified as “Company A” in the court document, is Begemot Ventures International, a company created by the two men in February 2015, according to corporate records filed in Washington.

According to the charging document, “Company A” was paid more than $1 million through an offshore Cypriot bank account to advise the Opposition Bloc and its members, including a “prominent Ukraine oligarch,” who is not named. Patten assisted the party and the oligarch by lobbying the U.S. government and providing public relations assistance in an effort to “influence United States policy,” which should have triggered a FARA registration, prosecutors said.

Patten has worked around Republican politics for years, and he came to specialize in representing foreign interests and political campaigns, particularly in former Soviet states. On his website, he describes himself as an international consultant.

Patten also had a yearslong business relationship with Cambridge Analytica, the political data firm that worked on Trump’s 2016 campaign. Starting in 2014, the firm used Patten as a consultant for election campaigns and other projects in Africa, Europe and the United States, according to a person with direct knowledge of his activities. During the same year, Cambridge Analytica sought to drum up business in Russia.

Cambridge Analytica closed down in May after revelations that it had improperly harvested personal data from tens of millions of Facebook users without their permission. The company has since come under scrutiny by the Justice Department, though it is unclear whether that investigation is connected to Mueller’s inquiry.

In 2014, the same year Patten worked as a consult for Cambridge Analytica, he also worked for one of the candidates running for mayor of Kiev. Later, he and Kilimnik shifted their attention to the Opposition Bloc, which emerged from the ashes of the administration of President Viktor Yanukovych, who fled Ukraine in February 2014 amid mass street protests of his government’s corruption and pivot toward Moscow.

Though Patten and Manafort both worked on behalf of the Opposition Bloc, people familiar with the political party said the men were not close, and their primary connection was through Kilimnik.

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