By Nathan Hodge and Emma Burrows, CNN

(CNN) --

Russia's Investigative Committee, a law-enforcement agency, said it had opened a criminal probe into their deaths.

Russian media, citing journalistic colleagues, described the slain individuals as members of three-person documentary film crew. The Russian Foreign Ministry said journalistic documents were found with the bodies identifying the men as Orhan Dzhemal, Alexander Rastorguyev and Kirill Radchenko.

The Center for Investigation Management, or TsUR, a Russian online news outlet, said in a Facebook post the three were in the country on an assignment to investigate the activities of Wagner, a Russian private military firm.

TsUR did not immediately respond to a message seeking further information.

Wagner has deployed mercenaries in Syria and Ukraine. Wagner was involved in a deadly encounter with US forces and their allies in Syria in February. The US airstrikes targeting ISIS injured several Russian military contractors working for Wagner.

The men were killed near the city of Sibut, about 185 miles north of the capital Bangui, the Russian Foreign Ministry said in a statement.

Sibut Mayor Henri Depele told Reuters the three were ambushed by armed men who opened fire on their vehicle. Depele told Reuters their driver survived.

"The Russian embassy in CAR, unfortunately, was not informed about the presence of Russian journalists in the country," the statement said. "Employees of our diplomatic mission are now in close contact with local law enforcement agencies and government agencies to discover all the circumstances relating to the deaths of Russian citizens."

Shadowy mercenary firms

Russia has relied on -- but never officially acknowledged the existence of -- mercenary firms in conflict zones in Syria and Ukraine.

Earlier this year, Maxim Borodin, a Russian reporter known for his investigations into Russian mercenaries in Syria, died after a fall from his apartment in Yekaterinburg.

Officials said there was no sign of foul play, but it cast a spotlight on the risks and threats sometimes faced by journalists in Russia. Borodin had been investigating Wagner's activities.