Special counsel Robert Mueller and his team have reportedly met multiple times with an Ukrainian politician who was in touch with Trump associates about a plan to resolve the conflict between Ukraine and Russia-backed rebels.

Ukrainian politician Andrii Artemenko said that he received “assorted questions” from FBI agents in several interviews about his “meetings, dealings, and the questions discussed with various levels of the American political establishment,” according to Politico.

“These included congressmen, senators and representatives of the White House administration,” he told Politico Friday after he had met with Mueller’s team.

Artmenko said that he will appear before a grand jury next month under oath and that he anticipates the Ukraine-Russia peace plan will be a topic of discussion at the grand jury testimony.

According to the New York Times, the peace plan was discussed with Trump’s personal attorney Michael Cohen and Trump’s business associate Felix Sater in 2017.

Cohen reportedly provided the plan to former national security adviser Michael Flynn, according to Politico, although Cohen says that never happened after dispensing with that plan. Flynn was later removed from his post at the White House after it was revealed he misled Vice President Mike Pence about his communications with Russians.

The peace plan, which involved using Kremlin-backed rebels to address the conflict in Ukraine, was subject to backlash because it considered leasing the Crimean Peninsula to Russia for 50 years in exchange for ending the war in Ukraine’s Donbass region.

Artemenko was removed from his political party after the proposal emerged, and Ukraine is investigating if he committed treason.