ABC News August 31, 2018

Sam Patten, Manafort associate, pleads guilty in case referred by Mueller

WATCH: Why a special counsel is investigating the Trump campaign's ties to Russia

Federal prosecutors on Friday targeted a veteran Republican lobbyist in a case referred by special counsel Robert Mueller, landing a guilty plea and cooperation from the political operative who illegally purchase tickets to President Trump’s inauguration on behalf of a foreign client, according to court papers.

The lobbyist, Sam Patten, pleaded guilty to acting “as an agent of a foreign principal, to wit, the Opposition Bloc (a Ukrainian political party) and its members, without registering” as a foreign lobbyist, according to court documents filed by the U.S. Attorney’s Office in Washington, DC.

As part of his plea agreement, Patten, 47, signed on to cooperate as a witness for the special counsel’s investigation into Russian meddling during the 2016 election, becoming the latest figure to fall under Mueller’s probe.

Patten’s cooperation with Mueller could be critical, given the longtime operative’s connections in the nation’s capital and around the world, including Paul Manafort, the former Trump campaign chairman, and Konstantin Kilimnik, a Russian national who was indicted in June.

MORE: Manafort hit with indictment for conspiracy to obstruct justice; former associate charged

Manafort was recently convicted of eight counts of financial crimes in a federal case brought by Mueller's team. Manafort's lawyer, Kevin Downing, said his client was "disappointed" with the verdict and "weighing his options."

Kilimnik has not entered a plea.

Prosecutors say Patten, who could face five years in prison and a fine of no more than $250,000 used a “straw” purchaser to secure tickets to President Trump’s inauguration in January of 2017, circumventing rules about the inauguration committee accepting money from foreigners.

Patten, through the Ukrainian oligarch for whom he worked, paid $50,000 for four tickets to the inauguration, the documents say.

He then "misled" the Senate Intelligence Committee about the inauguration ticket purchase in January 2018, leading the panel to issue a separate criminal referral to the Department of Justice, according to court documents filed Friday.

According to prosecutors, Patten launched a lobbying firm with an unnamed Russian national in 2015 and began working on behalf of the Ukraine’s Opposition Bloc –- a group for whom Manafort also lobbied -– travelling frequently to Ukraine and accepting payments through an offshore account in Cyprus, as Manafort had.

As part of his work for the Ukrainians, Patten contacted and attempted to meet with members of Congress and their staff in an effort “to promote the interests” of his clients in Ukraine, according to court documents.