Putin abandoned Russian mercenaries fighting in Syria, relatives say
The incident has been played down by both Moscow and Washington.
The group of men told friends they had found "a gig" before travelling to the southern Russian city of Rostov-on-Don and later to Syria.
"Who will help us? They left on their own, somebody came there to collect them and took them away. Where did they go?"
Her son had previously fought in two wars in Russia's Chechnya region but struggled to find work afterwards, Berdysheva said.
Russian guns for hire
The death of Russian citizens in Syria has exposed the role of Russian mercenaries in the multi-front conflict.
Russia can legally prosecute mercenaries under an existing law which has been applied against several citizens fighting in Ukraine and Syria in recent years.
In 2014, two Russian men, Vadim Gusev and Yevgeny Sidorov, were sentenced to three years in prison after they recruited over 200 former military soldiers to an outfit called the Slavonic Corps for a trip to Syria's Deir az-Zour.
According to Fontanka website, which has chronicled the involvement of private military contractors in Syria, the Slavonic Corps later became the core of a new mercenary group recruited by former member Dmitry Utkin, nicknamed "Wagner".
The Wagner group has no website or social networking page, instead attracting men with military experience through word of mouth.
Utkin and the Wagner group was blacklisted by the US Treasury in 2016 for having "recruited and sent soldiers to fight alongside separatists in eastern Ukraine".
According to Fontanka, Wagner is associated with a Russian company Yevro Polis, which has signed a deal with the Syrian government.
Under the deal, the company would capture and secure oil and gas infrastructure in Syria in exchange for a 25 percent share in future resource production.