– In the nearly two years that the special counsel, Robert Mueller, has been investigating whether there was collusion between Russia and the Trump campaign, few figures seem to have offered more tantalizing leads than Konstantin Kilimnik.

A diminutive, multilingual political operative who was born in Ukraine while it was still part of the Soviet Union, Kilimnik has continued to attract intense interest from prosecutors for his interactions with his longtime boss and mentor, Paul Manafort, and his suspected ties to Russian intelligence.

Kilimnik pops up repeatedly as a possible connection between the Trump campaign and Russia, with ties to both sides that are as enigmatic as they are deep. His dealings with Manafort, who in 2016 served as Donald Trump’s campaign chairman, encompass two of the most intriguing elements of the special ­counsel’s inquiry to surface publicly: the sharing of polling data with Kilimnik, and the work he and Manafort did on behalf of Kremlin-aligned Ukrainian interests.

Dozens of interviews, court filings and other documents show Kilimnik to be an operator who moved easily between Russian, Ukrainian and American patrons, playing one off the other while leaving a jumble of conflicting suspicions in his wake. To U.S. diplomats in Washington and Kiev, he has been a well-known character for nearly a decade, developing a reputation as a broker of valuable information.

He traveled freely to the United States, and on a trip in May 2016 met senior State Department officials for drinks. Later that year, he visited the new U.S. ambassador to Ukraine in Kiev. But in a federal court in Washington, Mueller’s prosecutors have repeatedly portrayed Kilimnik as something potentially more nefarious: “a former Russian intelligence officer” who “has ties to a Russian intelligence service and had such ties in 2016.”

And around the same time he was passing through Washington nearly three years ago — just as Trump was clinching the Republican nomination — he first received polling data about the 2016 election from two top Trump campaign officials, Manafort and Rick Gates, as Russia was beginning a social media operation intended to help Trump’s campaign.

Prosecutors have also scrutinized the effort by Manafort and Kilimnik to drum up political consulting business with Kremlin-aligned political figures in Ukraine and Russia who were pushing plans to end the simmering conflict between the countries. Those plans could have resulted in the easing of sanctions imposed on Russia by the United States — a policy shift to which Trump had signaled an openness during the campaign.

In many ways, Kilimnik is an unlikely figure for such a pivotal role in an investigation that has shaped Trump’s presidency. To some of those he encountered, he was an impish, 5-foot-tall cynic whose American associates nicknamed him “Carry-on” or “KK.”

At the same time, he did little to defuse long-running suspicions that he was a Russian agent. And his involvement in discussions related to back-channel peace negotiations between Ukraine and Russia attracted attention from President Barack Obama’s National Security Council, which saw him as a functionary for oligarchs working to sell out Ukraine to Moscow’s benefit, a former U.S. official said.

Kilimnik, 49, who has Russian citizenship, lives in Moscow. He is unlikely to ever face obstruction of justice charges that the special counsel brought against him and Manafort.

He studied at a Soviet military language academy considered a training ground for the country’s military intelligence service, known as the GRU. He joined the Russian army as a translator, and said he later worked in Sweden for a Russian arms exporting company.

In 1995, Kilimnik landed a job in the Moscow office of the International Republican Institute, or IRI, which receives tens of millions of dollars annually from the U.S. government to promote democracy.

Two former IRI colleagues have said the group fired Kilimnik in April 2005 after suspicions arose that he had leaked information about an institute conference in Bratislava, Slovakia, to Russia’s Federal Security Service, the successor agency to the KGB.

An institute spokeswoman declined to address the allegation, instead saying that Kilimnik was fired because the organization “came into possession of information that led us to believe that he was joining Paul Manafort’s team on the Ukraine program.” That was a “violation of our code of ethics” banning freelancing, the spokeswoman said.

Kilimnik quickly made himself indispensable to Manafort, who spoke neither Ukrainian nor Russian. At the time, Manafort worked as a political and business consultant to Ukrainian steel and coal oligarch Rinat Akhmetov, as well as Russian aluminum oligarch Oleg Deripaska.

“Paul took Konstantin under his wing, not just as a protégé but as his own surrogate son,” said Michael Caputo, who traveled in the same circles as Kilimnik.

Gates, who joined Manafort’s Ukraine team the year after Kilimnik and has since pleaded guilty to charges brought by Mueller, later told an associate he knew that Kilimnik was a former officer with the GRU, according to a court filing by Mueller’s team.

In a February 2017 interview with the New York Times in Kiev, Kilimnik denied ever serving in a Russian intelligence agency. “What would I do if I were a real Russian spy?” he said. “I would not be here. I would be in Russia.”

But for the politicians and oligarchs who were Manafort’s clients, Kilimnik’s suspected intelligence connections suggested a seal of approval from Russia. The perception in political circles in Kiev was that hiring Manafort’s team would open doors in Washington and Moscow.

Akhmetov persuaded Manafort to try to resuscitate the political career of former Ukrainian Prime Minister Viktor Yanukovych, a Russia-aligned figure who lost the 2004 presidential election amid allegations of vote-rigging.

With help from Manafort and Kilimnik, Yanukovych won the presidency in 2010. But things started going south for Yanukovych in late 2013, as mass protests erupted over the government’s corruption and pivot toward Russia. Yanukovych stepped down and fled in February 2014, eventually arriving in Moscow.

Russia began a military incursion into eastern Ukraine in 2014. And Kilimnik leveraged his access to U.S. diplomats for a new endeavor. He turned up in Donetsk as the city was slipping into war, presenting himself as having “the State Department’s ear, the American ear in general,” said Alexei Kovzhun, a member of a group of pro-Ukrainian political advisers working there.

With their primary benefactor in exile, Manafort and Kilimnik sought business with Russia-aligned factions that arose from the ashes of Yanukovych’s party. A common goal for these factions was to pursue initiatives to settle the Ukraine conflict on terms seen by many in the country as favorable to Russia, and as an opening for an effort to persuade the U.S. and its allies to lift sanctions on Moscow.

Among the clients they advised was a party backed by Serhiy Lyovochkin, a television company owner who served as Yanukovych’s chief of staff.

Around the time of Kilimnik’s trip to the U.S. in spring 2016, Manafort directed Gates to transfer some polling data to Kilimnik, including public polling and some developed by a private polling company working for the campaign, according to a person with knowledge of the arrangement. Manafort asked Gates to tell Kilimnik to pass the data to Lyovochkin and Akhmetov, the person said. Representatives for both Lyovochkin and Akhmetov said they neither requested nor received the data and would have had no use for it.

Mueller’s team has focused on what appears to have been another discussion about polling data in New York on Aug. 2, 2016. A partly redacted court transcript suggests that Gates, who entered a plea agreement with the special counsel that requires his cooperation, may have told prosecutors that Manafort had walked Kilimnik through detailed polling data at a meeting that day.

Prosecutors contend that Manafort lied to them about the meeting and other interactions with Kilimnik.

Just before Manafort was forced to resign from the Trump campaign in mid-August 2016 amid scrutiny of his work in Ukraine, the FBI opened an investigation into the campaign’s possible ties to Russia. By the inauguration, investigators were focusing on Kilimnik and his connections to Manafort. Yet Kilimnik again traveled to the United States for Trump’s inauguration, meeting in the Washington area with Manafort, according to Mueller’s team.

Manafort’s allies point out that Mueller’s team has not publicly presented any evidence that Kilimnik is a Russian agent.

“If he was a Russian intelligence asset, then the State Department officials who met with him over the years should be under investigation,” Caputo said.