A Russian-speaking man who filmed the torture and killing of a prisoner in Syria has been identified as a member of Wagner, a Russian private military company with ties to the Kremlin that has aggressively expanded into the Middle East and Africa in recent years.

The video, which appears to have been taken in 2017, showed several men in military-style clothing beating a prisoner with a sledgehammer, then beheading him, amputating his arms with a sapper’s trowel and setting his body on fire using gasoline or another accelerant.

The mercenary, who was been identified as Stanislav D by the Novaya Gazeta newspaper, is a former police officer from the Stavropol region in southern Russia. In leaked documents, he wrote that he joined Wagner in 2016 to “protect the interests of the Russian Federation abroad”.

The Kremlin said on Thursday it had no knowledge of the incident. “This has absolutely no relation to Russian soldiers, no matter what is being published about it,” said Dmitry Peskov, a spokesman for Vladimir Putin.

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The newspaper has not released the man’s full last name in order to protect his family from potential reprisals. Online messages sent to him and his wife by the Guardian were not immediately returned on Thursday.

The victim is reported to be Mohammad Taha al Ismail Abdallah of Deir ez-Zor, whom the contractors accused of deserting from the Syrian army. Family members had confirmed his death to local press, saying he had returned to Syria from Lebanon in 2017 when he was summoned to fight but later deserted.

Snippets of the video had been posted online since 2017 but the full footage was uploaded last week to a closed VKontakte group for members of Wagner. It was quickly seized upon by open source investigators, who first identified the location of the video as the Shaer gas station near Homs and then pored over the video for clues to the men’s identities.

The men, who appear to be drunk in the video, sought to hide their identities by covering their faces. But later they appear to let their guard down, as one says: “Hide your faces … well, whatever, this video won’t go up anywhere anyway.”

In the video, the men urge each other on to greater violence, telling one to “cut harder” while beheading the victim. Afterwards, the men posed for a photograph with the suspended body, across the torso of which they wrote: “For the VDV [Russian airborne forces]” and an out-of-focus word that appears to say razvedka, a term for military intelligence.

At one point in the video, a second severed head can be seen lying on the ground. That person has not been identified.

The incident will serve to further blacken the reputation of Wagner, which was founded by a businessman close to Vladimir Putin. Its mercenaries have also fought in Ukraine and the Central African Republic, and are said to be active in Libya, Sudan and Mozambique.

Thousands of Russian contractors fought in Syria alongside a 2015 military intervention by Putin that helped turn the tide in the country’s civil war and saved the government of Bashar al-Assad. But Wagner’s work, which has reportedly been rewarded with oil concessions in Syria, has largely taken place in the shadows, and the video is a rare, shocking look at the group’s actions in Syria.

Novaya Gazeta said it had first identified Stanislav D using online facial recognition software, then corroborated the results by showing he was a member of the private military company using a leaked copy of his passport. The newspaper said it had also obtained an application form he had filled out for Wagner and a copy of a non-disclosure agreement he had signed.

The open source investigation team Bellingcat also said it had identified the same person. The Guardian checked the results independently using the facial recognition software FindClone and found the same results.

Novaya Gazeta called on Russian law enforcement to investigate the case. But the Russian government has preferred to keep Wagner’s actions obscured, even hiding preparations for funerals when members of the private military company have been killed abroad.