Voronenkov’s first interviews were works in progress. Confronted with his social-media posts about Crimea, he said his Twitter account had been hacked. As for his vote on annexation, he started by denying he had voted yes, only to change course and say he had been forced to do it. Not everyone in the press bought his explanations, but he was slowly getting better, more polished in his presentation. The interview with Dvali, his last in the Ukrainian press, was indeed his best. Less than two hours after it was published, he was dead.

In September 2017, the Ukrainian prosecutor’s office held a news conference to announce the results of its investigation. Lutsenko, Kononenko and Ponomarev lined up in front of the cameras to announce that law enforcement had solved the case. Under F.S.B. direction, they explained, Vladimir Tyurin — Maksakova’s former partner — hired three Ukrainian radicals through his criminal network: the shooter, whose nom de guerre was the Boxer; an associate, called the Hunter; and the driver, Yaroslav Tarasenko, who was arrested in June but maintained his innocence. “The heads of the F.S.B., together with the heads of the criminal world of Russia, had prepared actions toward the murder — or elimination, from their point of view — of an incredibly valuable witness,” Lutsenko began. (The prosecutor’s office denied my request to speak to Tarasenko, but I traveled to his hometown and spoke to his mother, his girlfriend and friends, who maintain that the Ukrainian government is setting him up.)

A little more than a week later, I sat down with Ukrainian officials to understand how they had traced the chain from petty criminals in Ukraine to the ex-partner Russian mafia don to the F.S.B. They repeated Voronenkov’s story about the embittered general, Oleg Feoktistov, and his vendetta against Voronenkov. I wanted to understand if they really knew who Voronenkov was. When we spoke earlier in the summer, the Ukrainian Ministry of Internal Affairs spokesman described the operation as “Russian hands in Ukrainian gloves.” After the announcement, they were walking back their claim that it was a Kremlin hit, but only vaguely. I asked the prosecutor general, Lutsenko, for any evidence they had that the F.S.B. or Feoktistov had played any role. “We have proof only against Mr. Tyurin, concerning his personal participation in this conspiracy,” he said. “All the rest are theories, which I offered.”

But what separated this theory from all the other conspiracies swirling around the case? Did they have any proof? “We have no proof that they met and made arrangements, paid money or organized the purchase of weapons,” Lutsenko said.

Lutsenko repeatedly held up Voronenkov as a valuable witness in the Yanukovych trial. But a Ukrainian journalist shared a copy of his testimony with me. Neither of us found anything revelatory in it. The closest thing to a bombshell was a letter that Voronenkov had referred to — one sent from Yanukovych to Putin begging for troops in his fight to stay in power — but that same letter had already been waved around at the United Nations by Russia’s delegate. At the news conference announcing the results of the investigation, Lutsenko held firm that Voronenkov was a truth teller, citing his testimony in yet another case of Russian aggression in Ukraine. “Elimination of such a witness and in such a case could have surely resulted in a nexus of a criminal boss with the special services,” he told me. (Tyurin’s lawyer denied that his client had anything to do with the murder. “If he really wanted to kill Voronenkov, he would have done it while Denis and Maria were still living in Russia.” A request for an interview with Feoktistov sent to the F.S.B. was never answered.)

The Ukrainian prosecutor’s theory seemed to make sense, and yet when one looked at it closely, its underlying logic seemed to color, flare and disintegrate, like a paper held too close to a flame. It was a combination of things the investigators could prove and things they wished to be true, and it relied primarily on a host of assumptions about Russia that were advantageous for Ukraine to promote: Russia as a monolith, with all power flowing in and out of the Kremlin. This myth has flourished in part because everyone has an interest in promoting it: The Ukrainians get their villain; the Kremlin gets a reputation for fearsomeness and efficiency.

To the contrary, the story of Denis Voronenkov is about something else entirely: a story of the chaos at the heart of the sistema, the Darwinian chaos Voronenkov himself exploited, mastered and was ultimately felled by. The ambiguity bred by such bedlam now stood in the way of us ever really understanding who or what was truly responsible for his death. It was strange that Voronenkov was unable to find his way out of the building raid, uncharacteristic of him to get burned by something so simple — was it the sum of all the people he had double-crossed? Had someone he swindled hired an assassin to kill him? Maksakova herself did not believe the Ukrainian government’s theory. She thought that her husband had been killed in a hit that involved the wealthy businessman from the building raid, whose lawyer denied that he played any role.