He Says He’s an Innocent Victim. Mueller Says He’s a Russian Spy.

KIEV, Ukraine — The man sat at a restaurant table, grasping a glass of white wine. His sandy hair was close cropped, he wore a cardigan sweater and in the afternoon bustle he looked like just another office worker at lunch.

While seated, the most notable element of his appearance was hardly noticeable; only when he stood to introduce himself did it become clear that he is short, almost childlike, in stature, a characteristic that earned him the nickname “the midget" from Russian political operatives.

He spoke flawless English, with only a touch of an accent, was gregarious, and casually brushed aside the main question in this rare interview in Kiev, the Ukrainian capital, a year or so ago, saying that of course he was not a Russian spy.

Yet in Washington these days, the man, Konstantin V. Kilimnik, has turned up in multiple court filings by the special prosecutor, Robert S. Mueller III, who identifies him as Person A. Just this week, for example, a Dutch lawyer was sentenced to a month in prison for lying to the FBI about, among other things, his communications with Person A.

And last week, Mueller turned over a card in the investigation into the Trump campaign’s possible collusion with Russia by asserting in a court document that this person “has ties to a Russian intelligence service” and was in contact with a senior member of the campaign, Rick Gates, during the 2016 election.

“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, 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 Manafort, acting as a go-between and fixer for the U.S. and the Russian-leaning politicians who were its clients.

The Russian government has denied meddling in the 2016 election and President Donald Trump has denied collusion by members of his campaign staff. But during the years that Manafort worked in Ukraine, the country was deeply penetrated by Russian intelligence agents. While 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 GRU 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 FSB, the successor agency to the KGB, talked in a speech about the private meetings of the institute’s officials.

They didn’t have evidence, but suspected Kilimnik had been the source, said the former official, who could not be cited publicly discussing personnel issues.

In the interview, Kilimnik said he had been dismissed for having taken work on the side as an interpreter for Manafort in Ukraine in the early 2000s.

It is not known whether Manafort, a longtime consultant to Republican politicians, was aware of the suspicions of the institute’s managers when he hired Kilimnik in 2005. Manafort’s business in Ukraine was registered in Kilimnik’s name.

Manafort’s former client President Viktor F. Yanukovych was deposed in 2014, and Kilimnik said he stopped working for Manafort that year.

In August of 2016, 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 Kilimnik’s background in military intelligence deserved scrutiny.

“He was a student of a military school in Russia,” Ariev said. “Everybody in the former Soviet Union knows what that means. They produce professional spies.”

In person, though, 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 U.S. election, Kilimnik said, he and Manafort had spoken “every couple of months,” at a time when 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 Kilimnik had communicated with Gates late during the 2016 campaign, and that Gates was aware of Kilimnik’s background in Russian intelligence.

The filing was notable for touching on Gates’ activities during the campaign. He has pleaded guilty to lying to the FBI 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.

Gates’ communications with 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 FBI about his interactions with Kilimnik and with Gates. Kilimnik also played a role in a reported effort by Manafort to contact a Russian oligarch, Oleg V. Deripaska, during the campaign.

Manafort and Kilimnik had cooperated on an ultimately unsuccessful business venture financed by Deripaska, known as the Pericles investment fund.

In July, 2016, while Manafort was chairman of the Trump campaign, Manafort emailed Kilimnik asking him to offer Deripaska “private briefings” about the campaign in exchange for resolving a multimillion dollar financial dispute related to the business, according to The Washington Post. Deripaska has said he never received the offer. Kilimnik, reached by email, declined to comment on this matter and the special counsel’s court filings.

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 Kilimnik closely enough to know his nickname among Russian-leaning political operatives in Kiev.

At the time, about eight years ago, Akhmetshin was trying to persuade political advisers of Yanukovych to buy the rights to a book that cast a domestic political opponent in a negative light, and attended meetings with Kilimnik.

In the interview last year, 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, Mueller asserted that Kilimnik was now based in Russia. In email exchanges over the past year, 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,” Kilimnik wrote in an email last year. “I am simply a random casualty because of my proximity to Paul,” he said, referring to Manafort.

Asked in the interview about the allegation of ties to Russian intelligence agencies, Kilimnik said, “I vehemently deny it.”