>> reporter: cnn has tracked wagner mercenaries operating in the central african republic, sudan, libya, mozambique, mali, and syria as well as ukraine. >> prigozhin has by some accounts 25,000 men, many of whom are very battle-hardened. >> reporter: battle-hardened and brutal. human rights groups say wagner, which has recruited murderers and drug dealers from russian prisons to fight, has committed a series of human rights abuses, including allegedly murdering and torturing a -- and executing a wagner fighter who had defected. >> the videos of prigozhin and his people using sledgehammers to maim and kill people, so he understands the value, the propaganda value of coming across as a brutal, almost uncivilized kind of person because that's another kind of power that i think the russian population certainly at least