One of the more mysterious subplots in the Trump-Russian melodrama resurfaced on Monday, when Andrii Artemenko, a Ukrainian politician who helped feed a Kremlin-approved peace plan to the White House shortly after the inauguration, confirmed that he is scheduled to appear before Robert Mueller’s grand jury in Virginia on Friday. “I received the subpoena last week,” Artemenko told Politico, adding that he intended to cooperate with the special counsel’s request. To many, the revelation was unsurprising. “I don’t see it as a break in the investigation; I see it as a continuation,” Patrick Cotter, a former assistant U.S. Attorney and longtime white-collar defense attorney, told me. But to careful observers of Mueller’s probe, Artemenko’s involvement is also a bright red signal that the special counsel is building a case for collusion. “There’s a strong likelihood that he’s trying to look for quid pro quos that involve the exchange of either business favors or money to people in Trump’s orbit, or Trump himself, in exchange for policy shifts,” Tim O’Brien, the author of TrumpNation: The Art of Being the Donald, told me.

The details of this supposed quid pro quo remain entirely theoretical, of course. But Artemenko’s participation in the peace scheme may provide a clue. As The New York Times reported last year, one week before Michael Flynn resigned as Trump’s national security adviser, a sealed proposal was hand-delivered to his office by Michael Cohen, the president’s lawyer, outlining a Kremlin-backed plan for a peace treaty in eastern Ukraine, which Russian forces had invaded. According to the Times, the plan had first come to Cohen via his longtime Trump Organization associate Felix Sater, a Russian-born real-estate developer with ties to organized crime (a “career criminal,” O’Brien alleged), who reached out on behalf of Artemenko. Sater introduced Artemenko to Cohen, and the plan was then reportedly handed over to Flynn. (Cohen has denied delivering the proposal.) If such a peace plan had been approved by the Trump administration, it might have provided the president with reason to remove the sanctions on Moscow, resolving a major dispute between the two countries and allowing hundreds of billions of dollars to flow back into Russia.

What Trump or his associates might have stood to gain from this arrangement, personally, also remains theoretical. “Who doesn’t want to help bring about peace?” Cohen told the Times, when asked about the plan. Artemenko likewise described himself as a Good Samaritan. “A lot of people will call me a Russian agent, a U.S. agent, a C.I.A. agent,” he said. “But how can you find a good solution between our countries if we do not talk?”

Mueller’s interest in the episode, however, suggests there may be more to the story. “We know that the investigators have been looking at the relationship between Sater, and Cohen, and Flynn, and those particular strands of the Russian connection to the Trump organization,” said Cotter, who worked closely with Mueller investigator Andrew Weissmann while prosecuting organized crime cases in New York. “What this is is another strand. It’s another person who I believe the investigators suspect is, or has been in the past, acting as a de facto agent for the Russian government, having significant policy political communications with the Trump organization.” The sequence of events is also notable: Artemenko, Sater, and Cohen met at the Loews Regency Hotel just days before Flynn resigned, after it was revealed that he discussed lifting sanctions with then-Russian ambassador Sergey Kislyak. Artemenko’s proposal also reportedly outlined how Trump could lift U.S. sanctions on Moscow as part of a settlement of the Ukraine conflict—an agreement that would have involved allowing Russia to “lease” the Crimean peninsula for 50 or 100 years. The plan also called for the ouster of Petro Poroshenko, the pro-Western president of Ukraine.