A classified State Department assessment concluded in 2018 that Ukraine’s former Prosecutor General Yuri Lutsenko—who is at the center of the impeachment inquiry of President Trump—had allowed a vital potential witness for Special Counsel Robert Mueller, Konstantin Kilimnik, to escape from Ukraine to Russia, beyond the reach of the United States, after a federal grand jury in the US charged Kilimnik with obstruction of justice.

If Kilimnik had been available for questioning, he had the potential to provide invaluable information to investigators that might have shed light on one of the most consequential unresolved questions that the American people deserve an answer to: whether the former chairman to President Trump’s 2016 presidential campaign, Paul Manafort, and perhaps other aides to then presidential candidate Trump, conspired with Russia to aid Russia’s covert operations to intervene in the election to defeat Hillary Clinton and elect Trump. By allowing Kilimnik to escape to Russia, Lutsenko foreclosed any possibility that Kilimnik would ever be questioned by US law enforcement and intelligence agents.

In making their assessment, US officials interviewed Ukrainian office holders, prosecutors, and sources within the National Anti-Corruption Bureau of Ukraine (NABU), an anti-corruption agency that, like the FBI, investigates malfeasance, but does not have the authority to bring criminal charges. Created in 2015 with the support of European Union nations and the Obama administration, NABU has uncovered evidence implicating many of Ukraine’s most powerful political and business leaders and oligarchs in serious criminal conduct, only to see potential cases languish when referred to Lutsenko’s office. NABU officials also more specifically told the State Department that not only had Lutsenko allowed Kilimnik to flee to Moscow, but that he also thwarted several investigations of Manafort.

The State Department at the time did not look into the possibility that Lutsenko allowed Kilimnik to leave Ukraine at the encouragement of anyone associated with President Trump. One former senior US diplomatic official explained to me, “I don’t think anyone ever seriously considered it… because it would have seemed implausible.” In recent days, however, it has been disclosed that President Trump had pressured Ukraine’s new president, Volodymyr Zelensky, to order his new prosecutor general to investigate former Vice President Joe Biden and his son, Hunter, to benefit Trump’s 2020 reelection campaign; that he pressured Zelensky to reinstate Lutsenko (whom Zelensky had fired) to his former position, threatening to withhold almost $400 million in military and security aid to Ukraine. This same official told me that today “it hardly appears implausible anymore that people around the president would have asked Ukrainian officials to interfere” with the Mueller investigation; this same person now recommends that the House committee conducting its impeachment inquiry of President Trump expand its probe to investigate the matter of Kilimnik’s escape to Russia as well.

Kim Wehle, formerly an Assistant US Attorney, and a constitutional scholar and professor at the University of Baltimore Law School, told me that if President Trump helped Kilimnik avoid standing trial or testifying in the US, that might constitute obstruction of justice, as well as “high crimes and misdemeanors” bearing on impeachment as envisioned by the Founding Fathers: “To do anything to prevent a witness from testifying—here, by manipulating the massive power of your high office to protect yourself from [potential] criminal liability or impeachment jeopardy—might be an impeachable offense,” Wehle said.

For more than a decade, Kilimnik had been Manafort’s right hand man in Kiev. During that time, the two men collected tens of millions in fees as political consultants for Viktor Yanukovych, who served as the president of Ukraine from 2010 to 2014. Yanukovych advocated that his country sever ties with the United States and other Western nations, and align itself more closely with Vladimir Putin’s Russia. Manafort and Kilimnik also consulted with Yanukovych’s political party, the Party of Regions, and with Ukrainian and Russian oligarchs with close ties to Yanukovych and Putin.

According to evidence contained in the Mueller report, throughout 2016 Manafort shared confidential polling data from inside the Trump campaign with Kilimnik regarding four states Manafort presciently believed might go for Trump: Michigan, Wisconsin, Pennsylvania, and Minnesota. Manafort, in turn, instructed Kilimnik to turn over the polling data to a former client, Oleg Deripaska, a Russian oligarch who is close to Putin. During the course of his investigation, the special counsel also disclosed that Kilimnik himself was “a former Russian intelligence officer” who still “has ties to a Russian intelligence service and had such ties in 2016.”

A covert social media campaign by operatives working for Russia’s military intelligence service, the GRU, was then already working to defeat Hillary Clinton and elect Trump. Russian agents intensified their targeting of voters in those four states in the final days of the campaign. Trump went on to win the presidency by winning Michigan, Wisconsin, and Pennsylvania by a margin of fewer than 80,000 votes cast in all three of those states, out of a total of 136 million ballots cast in total across the country. (In Michigan, the vote was so close that if more than 5,400 had voted for Clinton, she would have won the state.)

Besides Manafort and Kilimnik, the special counsel had no other potential witness who could say whether the polling information was shared with Russia.

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After having been convicted of eight felony counts of money laundering, tax avoidance, and bank fraud in August 2018, and facing additional serious charges, Manafort agreed to cooperate with Mueller’s prosecutors in exchange for leniency. He pledged to “fully, truthfully, completely and forthrightly” answer any questions the special counsel’s investigators wanted to ask. (Manafort is currently awaiting trial on sixteen additional charges, including mortgage fraud, brought by the Manhattan District Attorney.)

But Manafort’s cooperation was a ruse. Only three months later, the special counsel asserted in federal court that Manafort had broken his cooperation agreement by telling prosecutors and FBI agents “multiple discernible lies.” Among those deceptions, the special counsel alleged, was that “Manafort lied to the [special counsel] and the grand jury concerning his interactions and communications with Konstantin Kilimnik about Trump Campaign polling.” That left Kilimnik as the only other living witness from whom the special counsel might have had a chance of compelling the truth.

In June 2018, Manafort and Kilimnik were indicted by a federal grand jury—with Kilimnik facing charges of witness-tampering and obstruction of justice. If convicted in a US court, Kilimnik faced years in prison. That would have given prosecutors leverage to compel Kilimnik to tell the truth about whether Manafort’s polling data was provided to Russian intelligence officials to help elect Trump. But when Lutsenko arranged for Kilimnik to slip across the border, he eliminated the last likely possibility for the American people to learn the truth.

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Classified State Department records say that in allowing Kilimnik to flee, Lutsenko was working with and at the direction of several senior officials in the administration of then-Ukrainian President Petro Poroshenko, who served from 2014 until 2019, when he was replaced by Zelensky. The records indicate that US officials believed it was highly unlikely that Lutsenko had acted without Poroshenko’s personal approval.

The State Department assessment noted that Ukraine was awaiting military aid from the Trump administration, and therefore Poroshenko was seeking favor with the Trump administration. Trump’s decision to withhold and delay congressionally approved military aid to Ukraine has now become a central matter of interest in the House impeachment inquiry. The State Department records further observed that Lutsenko had sought to act in the tradition of other Ukrainian prosecution, pursuing investigation to discredit and imprison enemies and protect his own political allies. That assessment is shared by the whistleblower whose complaint led to the impeachment proceedings. The whistleblower’s memo to the Intelligence Community’s Inspector General asserted that Lutsenko “has no legal training and has been widely criticized in Ukraine for politicizing criminal probes and using his tenure as prosecutor general to protect corrupt Ukrainian officials.”

It is unclear whether President Trump and Rudy Giuliani were aware of this information about Lutsenko when Giuliani and Lutsenko (according to Lutsenko) spoke or met about ten times, and President Trump pressed Ukraine’s new president, Volodymyr Zelensky, to reinstate Lutsenko after Zelensky had fired him. But there was an extensive public record regarding what Lutsenko had done before Giuliani had first spoken to Lutsenko. On May 2, 2018, Andrew Kramer of The New York Times, who works from the newspaper’s Moscow bureau, reported that Ukrainian officials had shut down four Manafort-related potential cases—each one having been “effectively frozen by Ukraine’s chief prosecutor.” Kramer explained:

The cases are just too sensitive for a government deeply reliant on United States financial and military aid, and keenly aware of Mr. Trump’s distaste for the investigation by the special counsel, Robert S. Mueller III… The decision to halt the investigations by an anticorruption prosecutor was handed down at a delicate moment for Ukraine, as the Trump administration was finalizing plans to sell the country sophisticated anti-tank missiles, called Javelins.

In May 2018, three Democratic US senators, Patrick Leahy of Vermont, Richard Durbin of Illinois, and Robert Menendez of New Jersey, alarmed by Lutsenko’s decision to freeze several investigations related to the Ukrainian dealings of Trump’s former campaign chairman Paul Manafort, wrote to Lutsenko:

We are writing to express great concern about reports that your office has taken steps to impede cooperation with the United States Special Counsel Robert Mueller… This reported refusal to cooperate with the Mueller probe also sends a worrying signal—to the Ukrainian people as well as the international community.

Lutsenko made no reply, but the State Department cable traffic provides an affirmative answer, finding that Lutsenko had indeed worked to thwart the special counsel’s work. Consistent with this record is the fact that Giuliani found Lutsenko accommodating of Trump’s wishes.

During a meeting in January with Giuliani, Lutsenko agreed to direct his office to take steps to reopen investigations into a Ukrainian oligarch who had employed Hunter Biden—part of Giuliani’s effort to tarnish the image of a potential rival to Trump in the 2020 presidential race.

Giuliani was reportedly so excited by Lutsenko’s favorable response that he immediately called President Trump to tell him about what had just transpired. Giuliani and Lutsenko arranged for Lutsenko to be interviewed by journalists for pro-Trump outlets, to whom Lutsenko broke the news that he was investigating the Bidens. Soon, this news was amplified on Fox News, with Giuliani and Trump themselves both appearing at various times to out the story.

It was a setback for Giuliani and Trump, then, when Zelensky signaled in April that, if elected, he planned to fire Lutsenko from his post as prosecutor general.

He waited only until July to do so, after parliamentary elections had reinforced his mandate. Throughout the State Department and US intelligence agencies, Lutsenko’s firing was viewed as an important first step of reform after the corrupt Poroshenko administration. The State Department, after all, knew that Lutsenko was not only helping Giuliani and Trump, but had also frustrated Mueller’s investigators and helped Kilimnik and, thereby, Manafort, too.

Thus, in his July 25 phone call with Zelensky, Trump pressured him to reinstate Lutsenko, whom Zelensky had removed, and allow him to continue his investigation of the Bidens and other political adversaries of the president. Just prior to the phone call, President Trump decided to hold back almost $400 million in military aid to Ukraine.

Trump also complained to Zelensky about Lutsenko’s firings: “I heard you had a prosecutor who was very good and he was shut down and that’s really unfair. A lot of people are talking about that, the way they shut your very good prosecutor down and you had some very bad people involved.”

“I wanted to tell you about the prosecutor,” Zelensky replied. “First of all I understand and I’m knowledgeable about the situation. Since we have won the absolute majority in our parliament, the next prosecutor will be 100 percent my person, my candidate, who will be approved by the parliament and will start as a new prosecutor in September.”

It was evident from the memorandum of the July 25 conversation that Zelensky was extremely careful to show deference toward Trump and placate him wherever possible. But on this matter, tellingly, Zelensky asserted himself and held absolutely firm.

In their letter to Lutsenko in 2018, the three US Senators wrote: “Ours is a relationship built on a foundation of respect for the rule of law and accountable democratic institutions.” The Senators lectured the Ukrainian prosecutor that by allowing for the politicization of their criminal justice system, especially to protect their political allies and undermine their political adversaries “sends a worrying signal” to their own “people as well as the international community.” They could have just as easily been writing about their own country.

An earlier version of this article suggested that Konstantin Kilimnik faced possible extradition to the US; this was a theoretical possibility if Kilimnik had traveled to a country like Austria that has an extradition treaty with the United States, but Ukraine does not have such an extradition agreement with the US. The article has been updated to clarify this point.