The murders of three Russian journalists investigating the work of private military contractors in the Central African Republic are still shrouded in mystery.

But the exiled Russian billionaire who was funding their work says evidence gathered so far indicates they were not the victims of a random attack but a well-planned ambush.

The three journalists -- Kirill Radchenko, Alexander Rastorguyev and Orkhan Dzhemal -- were shot dead last month on a remote road in the north of the CAR. They'd gone to the country to investigate the presence of Russian military contractors linked to Wagner, a company close to the Kremlin that is also active in Syria and Sudan.

Mikhail Khodorkovsky, the exiled Russian billionaire whose foundation supported the investigation, told CNN that "some people who we know in Russia were approached by people connected with Russian mercenaries, and said they had warned the journalists not to go there, to keep themselves well away from the area."

He said that his foundation was continuing to investigate the murders but the "only reasonable version is that either they had already learnt something or that they could have already discovered something ... We think the version of a simple robbery is totally unsustainable."

People who had been to the site said it looked like the assailants had been waiting there the whole day, Khodorkovsky said.

One man who might be able to cast light on what happened is the local driver -- a former policeman -- hired by the journalists. The Russian Foreign Ministry says that local authorities in CAR have interviewed him, and a Russian representative in CAR said the man was in custody in the capital.

Khodorkovsky accused CAR authorities of "hiding the driver, who is a key witness."

Alain Nzilo, a journalist in CAR who has followed the case, told CNN last week he found it strange that the assailants appeared to have let the driver go free. "Often, the drivers are the first to be killed and not the foreigners," he said.

Also unknown: why did the journalists insist on continuing their journey after dark after being advised by local security forces not to? And why did they change their route at the last moment, after leaving the town of Sibut?

According to a Russian official in CAR, they "had a word with someone and then changed their route." Khodorkovsky says: "This is impossible to explain on the basis of information which we possess."

•Flowers are left in front of photographs of Russian journalists, from left, Alexander Rastorguyev, Kirill Radchenko and Orkhan Dzhemal in a picture taken at Moscow's Central House of Journalists in August. The three men were killed in the Central African Republic in July.

Update news