“The Federal Bureau of Investigation special agents assisting the Special Counsel’s Office assess that Person A has ties to a Russian intelligence service and had such ties in 2016,” the filing said.
As Person A, Mr. Kilimnik, a 47-year-old former Russian military interpreter, has appeared now in multiple court filings by the special prosecutor, which suggests that he could become a pivotal figure in the investigation. For about a decade, he worked as an office manager in Kiev for the political consulting business of Mr. Manafort, acting as a go-between and fixer for the American and the Russian-leaning politicians who were its clients.
The Russian government has denied meddling in the 2016 election and President Trump has denied collusion by members of his campaign staff. But during the years that Mr. Manafort worked in Ukraine, the country was deeply penetrated by Russian intelligence agents. While Mr. Kilimnik continues to deny that he was a Russian agent, it would have been perfectly normal for Moscow to plant someone in the Manafort operation.
Konstantin Viktorovich Kilimnik was born in eastern Ukraine in the Soviet period. He studied at the Military Institute of the Ministry of Defense in Moscow, and after the Soviet breakup took Russian citizenship, he said in the interview. The institute trains interpreters for the Russian military intelligence agency, formerly known as the G.R.U. and now called the Main Directorate.
He worked for a time in Sweden as an interpreter for a Russian company that exported arms, and later in the Moscow office of the International Republican Institute, a Washington-based nonprofit, where former employees said they suspected he was informing on them to the Russian authorities.
He parted ways with the organization, a former employee of the Moscow office said, after the chief of the F.S.B., the successor agency to the K.G.B., talked in a speech about the private meetings of the institute’s officials.
They didn’t have evidence, but suspected Mr. Kilimnik had been the source, said the former official, who could not be cited publicly discussing personnel issues.
In the interview, Mr. Kilimnik said he had been dismissed for having taken work on the side as an interpreter for Mr. Manafort in Ukraine in the early 2000s.
It is not known whether Mr. Manafort, a longtime consultant to Republican politicians, was aware of the suspicions of the institute’s managers when he hired Mr. Kilimnik in 2005. Mr. Manafort’s business in Ukraine was registered in Mr. Kilimnik’s name.
Mr. Manafort’s former client President Viktor F. Yanukovych was deposed in 2014, and Mr. Kilimnik said he stopped working for Mr. Manafort that year.
In August of 2016, Mr. Kilimnik was formally investigated in Ukraine on suspicion of ties to Russian spy agencies, according to documents from Parliament and the Prosecutor General’s Office, but no charges were filed.
A Ukrainian lawmaker, Volodymyr I. Ariev, who requested the investigation, said Mr. Kilimnik’s background in military intelligence deserved scrutiny.
“He was a student of a military school in Russia,” Mr. Ariev said. “Everybody in the former Soviet Union knows what that means. They produce professional spies.”
In person, though, Mr. Kilimnik has been surprisingly nonchalant about the suspicions swirling around his past and role in the 2016 campaign.
He said he was never contacted by investigators in Ukraine and called the probe politically motivated. “If there were any truth to me talking to any security service in the world, they would arrest me,” he said, speaking of Ukrainian law enforcement.
Before the United States election, Mr. Kilimnik said, he and Mr. Manafort had spoken “every couple of months,” at a time when Mr. Manafort served as chairman of the Trump campaign, but he said there was nothing to hide in the calls and meetings. The two mostly discussed Ukrainian politics, not the election, he said: “I was briefing him on Ukraine.”
The filing last week by the special counsel’s office asserted that Mr. Kilimnik had communicated with Mr. Gates late during the 2016 campaign, and that Mr. Gates was aware of Mr. Kilimnik’s background in Russian intelligence.
The filing was notable for touching on Mr. Gates’s activities during the campaign. He has pleaded guilty to lying to the F.B.I. and conspiring to defraud the United States for activities related to his work in Ukraine mostly before joining the Trump campaign, and agreed to cooperate with the investigation.
Mr. Gates’s communications with Mr. Kilimnik were revealed in the sentencing documents of a former lawyer for with the law firm Skadden, Arps, Slate, Meagher & Flom, Alex van der Zwaan, who pleaded guilty to lying to the F.B.I. about his interactions with Mr. Kilimnik and with Mr. Gates.
Mr. Kilimnik also played a role in a reported effort by Mr. Manafort to contact a Russian oligarch, Oleg V. Deripaska, during the campaign.
Mr. Manafort and Mr. Kilimnik had cooperated on an ultimately unsuccessful business venture financed by Mr. Deripaska, known as the Pericles investment fund.
In July, 2016, while Mr. Manafort was chairman of the Trump campaign, Mr. Manafort emailed Mr. Kilimnik asking him to offer Mr. Deripaska “private briefings” about the campaign in exchange for resolving a multimillion dollar financial dispute related to the business, according to the Washington Post. Mr. Deripaska has said he never received the offer. Mr. Kilimnik, reached by email, declined to comment on this matter and the special counsel’s court filings.
Mr. Kilimnik has surfaced as a fringe figure in other aspects of the Russian investigation.
Rinat Akhmetshin, a Russian-American lobbyist who attended a Trump Tower meeting with Donald Trump Jr. in June of 2016 where a Russian lawyer had promised to provide negative information on Hillary Clinton, had also worked in Ukraine with Mr. Kilimnik closely enough to know his nickname among Russian-leaning political operatives in Kiev.
At the time, about eight years ago, Mr. Akhmetshin was trying to persuade political advisers of Mr. Yanukovych to buy the rights to a book that cast a domestic political opponent in a negative light, and attended meetings with Mr. Kilimnik.
In the interview last year, Mr. Kilimnik said he divided his time between Kiev, where he worked, and Moscow, where his wife and two daughters lived in the suburb of Khimki.
In a court filing last year, Mr. Mueller asserted that Mr. Kilimnik was now based in Russia. In email exchanges over the past year, Mr. Kilimnik has declined to say where he is.
“I do not want to be part of the U.S. political games and I am not,” Mr. Kilimnik wrote in an email last year. “I am simply a random casualty because of my proximity to Paul,” he said, referring to Mr. Manafort.
Asked in the interview about the allegation of ties to Russian intelligence agencies, Mr. Kilimnik said, “I vehemently deny it.”