Blackwater, the private security firm now known as Xe Services, has acquired a controversial reputation in recent years for playing a largely unregulated, mercenary role as the US' former guard of choice in Iraq and other countries.

The group operated under a $465m contract in Iraq and became infamous for a September 2007 shooting in Baghdad's Nissour Square that left 17 civilians dead. Five Blackwater guards were charged with murder, though a judge in January dismissed the case for prosecutorial misconduct.

The WikiLeaks Iraq war files reveal 14 other incidents in which Blackwater guards opened fire on civilians, killing 10 and wounding seven.