Special Counsel Robert Mueller’s office has indicted a 20th person in its probe into Russian meddling and collusion in the 2016 presidential election.

Konstantin Kilimnik, a Russian political operative, was indicted for conspiracy to obstruct justice and obstruction of justice. Former Trump campaign manager Paul Manafort also received a new charge of obstruction of justice related to alleged witness tampering, his sixth charge.

“A federal grand jury in the District of Columbia has returned a third superseding indictment today against Paul J. Manafort, Jr., 69, of Alexandria, Va., which adds Konstantin Kilimnik, 48, of Moscow, Russia, as a defendant and charges both defendants with conspiracy to obstruct justice and obstruction of justice,” the Special Counsel said in a statement.

Mueller’s office has already charged Manafort with conspiracy against the United States, conspiracy to launder money, unregistered agent of a foreign principal, false and misleading FARA statements, and false statements. He has pleaded not guilty, and his trial is expected to begin in July. Earlier this week, Mueller’s office accused Manafort of witness tampering, alleging that he and an associate – only identified as “Person A” – tried to get two witnesses to lie about lobbying work for Ukraine. Mueller’s office has referred to “Person A” in previous court filings; the New York Times has confirmed it is Kilimnik.

Kilimnik, once described as “Manafort’s Manafort,” worked for his political consulting firm, Davis Manafort Partners International in Kiev, Ukraine. Prosecutors believe he has ties to Russian intelligence. The indictment alleges that in 2012, Manafort and his associate Rick Gates, who Mueller’s office also indicted last October for money laundering and failing to register as a foreign agent, worked with Kilimnik to lobby on behalf of former Ukrainian President Viktor Yanukovych to retain a group of lobbyists to act as independent assessors of the Ukrainian government, even though that government was actually paying them.

The obstruction of justice charges filed Friday allege that Kilimnik and Manafort attempted to influence two people – who were not explicitly identified in the filing – between February and April of 2018.

Renato Marriotti, a former federal prosecutor, said this indictment has potential to be politically significant, because it is the first one where a Russian and an American have been indicted together, the strongest evidence yet for potential cooperation between the two parties. “We have an indictment of a Russian and an American working together for the first time,” he said. “This is an indictment of the former chair of the Trump campaign for conspiring with a suspected Russian intelligence operative. I think there’s been a narrative for a while that they’re waiting to find collusion … conspiracy seems very similar to collusion to me. Thats about as close of an analogue as you’re going to find in the law.”

Manafort is a longtime acquaintance of President Trump – he has a condo in Trump Tower – but was only officially with the campaign for less than six months before he was ousted. However, during his tenure he was given the monumental task of trying to prevent an insurgency against Trump at the 2016 Republican convention, when Trump was the presumptive nominee but certain factions in the Republican party were trying to figure out a way to replace him with another candidate.

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Write to Alana Abramson at Alana.Abramson@time.com.