From this:

Mercenaries believed to be from Chechnya are suspected of carrying out the deadly ambush of civilian security contractors on a road outside Baghdad earlier this month.

The attackers killed four civilians working with Blackwater USA. Three contractors who managed to escape concluded that the assault was the work of trained gunmen...

Supposedly, two or more of those on the Berg tape said something in Russian.

I don't know why two Chechens would speak Russian to each other, but that could be a lingua franca for two people from different parts of the FSU.