I spoke by phone with the 74-year-old grandmother of Voronov, who was 23 when he died, a soldier in the special forces. He had lost both his parents at a young age, and his grandmother raised him for most of his life. In June, he told her he was headed to Rostov for exercises. On the morning of July 14, a small group of military officers showed up at her house in a rural village outside of Nizhny Novgorod and told her, as she remembers, “Your grandson no longer is.” She doesn’t know the details of his death; all she was told was that it was “due to military circumstances.”

Ruslan Garafeev — the fellow soldier with whom Petr claims, in the video released by the S.B.U., to have crossed into Ukraine — is most likely another fatality. A number of the independent websites that track cases of Russian soldiers in Ukraine list Garafeev as dead, although without confirmation. (Garafeev’s social-media accounts have been deleted, but before they were, his status read, “I’m rubbing out khokhly” — a disparaging Russian term for Ukrainians — “in Rostov.”) A former soldier who served with Garafeev told me that, according to what he was told by friends still in the unit, Garafeev had indeed been killed. (He said he was told of a zinc coffin containing Garafeev’s body.) This friend said it seemed believable that Garafeev would do something like join up with the rebels in search of easy money. “It wasn’t that he was a bad guy; he just didn’t think seriously about the consequences of things,” he told me. “Wherever the wind is blowing, that’s where he’ll fly. He heard he could get some good money, the wind blew in that direction and so he flew over there.”

Perhaps no one knows more about how post-Soviet Russia prosecutes its wars than Valentina Melnikova, the head of the Union of the Committees of Soldiers’ Mothers of Russia. Her organization came to prominence during the first war in Chechnya, in the mid-1990s, which began with a disastrous series of clandestine exercises in which scores of Russian soldiers were killed. Yet President Boris N. Yeltsin spent months denying that there were any Russian troops in Chechnya. In Moscow this fall, I went to visit Melnikova, a forceful 68-year-old with arched eyebrows and a short, tousled mop of red hair. The amount of paperwork stacked on every surface and spare corner of her office testified to the deadening complexity of the Russian bureaucratic machine. Every request for information she files requires letters and documents with official stamps; the answers, when they come, pile up in precarious towers around the committee’s one-room headquarters.

What had her the most confused and depressed was how mothers of servicemen deployed during the war in Chechnya would wait in lines as long as 200 people to get into her office. They wanted to know where their sons were and how to get them back. Now she gets a handful of inquiries from family members about Ukraine. “It’s like they don’t want to believe, or can’t believe, that it’s real life, that it’s really happening,” she said. Society had become passive again, too fearful or timid to take on the state: If the powers that be said there was no war, then there wasn’t.

Sergey was one of the few who had asked Melnikova for help, and she agreed to make some calls and put in official requests for information to see what she could learn about Petr’s fate. But despite her connections with military sources, Melnikova, who had been looking into the case for more than a month, still didn’t have any concrete answers. An official at the Defense Ministry told her that Petr was the subject of a criminal case for desertion; although, oddly, he added that it was the Russian state’s position that Petr went to fight not with the rebels but with the Ukrainian Army. Her appeals for information were circulating from one office to another. The state had decided to “play the fool,” she said, shuffling her requests from desk to desk in an endless loop. “It’s a dead end.”

She thought it was possible that Petr had been recaptured by Russian officers and was being held somewhere on Russian territory to keep him from talking. There was precedent for such a situation: In 1999, during the second war in Chechnya, Melnikova was called to an airport in Moscow to meet a group of Russian soldiers who had been captured by militants and then freed. But the soldiers never showed up; they were taken away by a special counterterrorism unit at the arrival gate, and it took Melnikova two weeks of calls to track them down. “We found them alive, but that was a different time,” she said. As for Petr, she said, “I don’t understand what kind of condition we’ll find him in, or if we’ll find him at all.” She shook her head. “It’s like it was in Chernobyl,” she said. “Everything has been covered by a giant sarcophagus.”

After a few days in Kiev, my own search for Petr finally yielded a lead. An S.B.U. official and representatives of the rebel forces all agreed that on Sept. 21, just before dusk on a stretch of highway outside Donetsk, Petr — along with 22 others suspected of being rebel fighters — was handed over from Ukrainian custody to the control of the separatist leadership. It was one of a handful of prisoner exchanges that took place this fall, as called for by the cease-fire accords signed in Minsk. I watched a video of the release, filmed by a member of the “politburo” of the rebel defense ministry. It all happens quickly: A bus pulls up and then Petr and the others get off and are lined up by the side of the road. A few minutes later, with the sun dipping below the horizon, they step onto another bus and head into rebel territory.