Russian filmmaker Alexander Rastorguyev was killed along with cameraman Kirill Radchenko and war correspondent Orkhan Dzhemal in the Central African Republic while filming an investigative documentary about the Wagner group, a private Russian military firm allegedly linked to Russian President Vladimir Putin, the Reuters news agency reported.

The three journalists, who were commissioned by Russian online news organization Investigation Control Centre to make the documentary, died in an ambush by unidentified assailants on Monday, Reuters said, quoting local authorities. The bodies were found in a bullet-ridden car near Sibut (about 300 kilometers from Bangui, the capital), according to the U.N. Mission in the Central African Republic.

“According to the driver’s explanations, when they were 23 kilometers [14 miles] from Sibut…armed men emerged from the bush and opened fire on the vehicle. The three journalists died instantly,” Henri Depele, the mayor of Sibut, told Reuters.

Russia’s foreign ministry issued a statement Tuesday identifying the victims and said that Russian diplomats were working with CAR officials to find out what happened.

The Investigation Control Centre was founded by Mikhail Khodorkovsky, an exiled oil tycoon and critic of Putin’s government.

“The crew were working in cooperation with my project The Centre for Investigation on an investigation into Russian private mercenaries, in particular the Wagner group,” Khodorkovsky wrote on his website. “These were brave men who were not prepared simply to collect documentary material, but wanted to ‘feel’ it in the palms of their hands. I hoped until the last moment that they had been captured, and that they could be rescued.”

Several media outlets have highlighted the controversial nature of the documentary the three men were making in the region. The Wagner group has reportedly been used by Russian authorities to conduct clandestine military operations on their behalf. Russian authorities have denied those reports, according to Reuters.

The Wagner group is headed by Yevgeny Prigozhin, who has known Putin since the 1990s and provides food services to the Kremlin through one of his companies, according to Bloomberg.