A local website published a photograph showing the bloodied corpses of two of the men in the back of a truck.
The men were making a film for TsUR, an investigative media centre funded by Mikhail Khodorkovsky, a former oligarch who was imprisoned for a decade after publicly disagreeing with the Russian leader, Vladimir Putin.
"I will make efforts to identify those responsible," Khodorkovsky, who now lives outside Russia, wrote on social media, giving no further details on what he intended to do.
Their topic was the Wagner group, a Russian private military contractor, according to TsUR's deputy chief editor, Anastasiya Gorshkova.
Dmitry Utkin, who was sanctioned by the US as Wagner's leader, has been photographed with Putin and was employed by Yevgeny Prigozhin, a catering magnate known as "Putin's chef".
Prigozhin was indicted by a US jury in February for running a Russian internet troll factory.
A Foreign Ministry spokeswoman denied that the journalists' death was connected to their investigation of Wagner. Russian state media have avoided mentioning the group in reporting the incident.
Russia has taken on an official role in the Central African Republic since December last year, when it was authorised by the UN to provide the armed forces with weapons and training.
But researchers from the Moscow-based Conflict Intelligence Team and Transparency International have raised suspicions that Wagner was guarding diamond mines in rebel territory.
The Foreign Ministry said Russia and the Central African Republic began joint "exploratory mining concessions" this year, but did not specify where.