( CNN) At the end of January this year, a giant Ilyushin-7 6 shipment aircraft touched down at the airport in Bangui, capital of the Central African Republic. It was the first of nine flights that brought tons of Russian weapons to bolster CAR's fragile government.

At the same time, 175 military trainers — all but a handful of them private contractors — arrived from Russia.

In turn, those trainers were followed by Russian companies eager to exploit the Republic's reputed mineral wealth.

The activities of those firms, which have ties to the inner circle of Russian President Vladimir Putin, were the target of three independent Russian journalists who were killed in CAR on Sunday: Kirill Radchenko, Alexander Rastorguyev and Orkhan Dzhemal.

Their trip was backed by a foundation — the Center for Investigation — run by Russian exile Mikhail Khodorkovsky. A long-time foe of Putin who spent years in prison in Russia, Khodorkovsky wrote on Facebook Wednesday that the journalists were working on a project about “Russian mercenaries.”

The deaths of the journalists have focused attention on Russia's growing interest in central Africa, and the relationship between the Kremlin and private Russian companies that combine security work with mining and other activities.

An ill-fated journey

The three humen had traveled via Morocco on tourist visas, informing neither the Russian embassy nor CAR authorities of their presence because they wanted to investigate the activities of a Russian military contractor called PMC Wagner, the Center for Investigation said.