Syria war: Who are Russia's shadowy Wagner mercenaries?

Russian mercenaries are reported to be in the thick of the fighting in Syria, helping President Bashar al-Assad's forces.

One shadowy group in particular, called Wagner PMC (Private Military Company), hit the headlines because of a clash on 7 February that resulted in dozens of Russian casualties.
The toll of dead and wounded is disputed, but it is still not clear why Russian irregulars attacked a base held by Kurdish anti-Assad forces and where US advisers were present. US forces retaliated with air strikes.

In June 2017 the US Treasury added Wagner PMC to a long list of Russian individuals and entities subject to sanctions because of their involvement in the Ukraine conflict.
The PMC "has recruited and sent soldiers to fight alongside separatists in eastern Ukraine", the US Treasury said.
The US also identified Dmitry Utkin as Wagner's "founder and leader" and placed him on the list. 

Source: Yahoo News