MOSCOW — Before he was killed by a sniper in Syria at age 23, Ivan Slyshkin wrote a poignant message on social media to his fiancee: “We will see each other soon – and I will hold you as tight as I possibly can.”

But Slyshkin’s name won’t be found among the Russian Defense Ministry’s official casualties in the fight against Islamic State extremists.

That’s because the young man who left his hometown of Ozyorsk in the Ural mountains was one of thousands of Russians deployed to Syria by a shadowy, private military contractor known as Wagner, which the government doesn’t talk about. Slyshkin’s gravestone depicts him holding a machine gun, according to a local news website Znak.com that sent a reporter to his March 2 funeral in Ozyorsk, where friends said he joined Wagner to earn money to pay for his wedding.

“He was in Wagner’s group,” his friend Andrei Zotov told The Associated Press, adding that Slyshkin was killed as the security forces were advancing on the Al-Shayer oil field north of Palmyra.

“There are many good guys there. He volunteered to join the company,” Zotov said. “Like many Russian fighters, he wanted to solve his money issues.”

The St. Petersburg-based website Fontanka reported that about 3,000 Russians under contract to the Wagner group have fought in Syria since 2015, months before Russia’s two-year military campaign helped to turn the tide of the civil war in favor of Syrian President Bashar Assad, a longtime Moscow ally.

When Putin went to a Russian air base in Syria on Monday and told Russian troops that “you are coming back home with victory,” he did not mention the private contractors. Russian troops are expected to remain in Syria for years while the contractors are likely to stay to guard lucrative oil and gas fields under a contract between the Syrian government and another Russian company allegedly linked to a businessman known as “Putin’s chef” for his close ties to the Kremlin.

Proxy fighters like Slyshkin have played a key role in Syria. In addition to augmenting troops officially sent by Moscow, their secret deployment has helped keep the official Russian death toll low as Putin seeks re-election next year.

The Russian Defense Ministry has said 41 of its troops have died in Syria. But according to Fontanka, another 73 private contractors have been killed there. The Kremlin and the Defense Ministry have stonewalled questions about Russians fighting in Syria in a private capacity. Private contractors have been used by countries like the United States in Iraq and Afghanistan for years; Russian law forbids the hiring of mercenaries or working as one.

But Russia has used such proxies before – in the conflict to help pro-Moscow separatists in eastern Ukraine since 2014. One Russian commander boasted of working alongside Russian troops who said they were “on vacation” while fighting in Ukraine.

The Defense Ministry has refused to say how many of its troops are in Syria, although one estimate based on absentee ballots cast in the Russian parliamentary election last year indicated 4,300 personnel were deployed there. That number probably rose this year because Moscow sent Russian military police to patrol “de-escalation zones.”

“The Russian people are not very enthused by the idea of an empire that would involve their boys coming home in body bags. There’s clearly a lack enthusiasm for this conflict,” said Mark Galeotti, senior researcher at the Institute of International Relations in Prague.

“By having this military company Wagner, they can have a force they can actually deploy … but when people die, it doesn’t have to be announced,” Galeotti said.

Fontanka, which is respected for its independent reporting, has obtained what it described as Wagner’s spreadsheets and recruitment forms that indicate thousands of Russians have fought in Syria. Of about 3,000 Wagner employees deployed to Syria over the years, the single largest contingent at a given time has been about 1,500, said Denis Korotkov, a reporter for Fontanka. Since 2015, at least 73 of them have died, he said.

Another investigative group, Conflict Intelligence Team, or CIT, put the number of private contractor deaths at 101. Both outlets say those are conservative estimates.

“The most important proof is people, dead and alive, who have said they are mercenaries and their relatives say there are mercenaries,” said CIT founder Ruslan Leviev. “How would hundreds of people all over the country collude and come up with the same story?”

Activists with CIT made a name for the group by combing social media and other records for Russia’s involvement in Ukraine and are doing the same for Syria. Both Fontanka and CIT published photos from what they called a Wagner training base in the Krasnodar region of southern Russia.

Agreements signed with the security companies have kept the private contractors and members of their families from speaking to the media about their activities. Survivors receive generous compensation for keeping silent, and most attempts by AP to contact relatives and friends of those killed have been unsuccessful.