The entire article is worth reading but the fact our military does not like working with these private contractors caught my attention!
http://www.brookings.edu/views/op-ed/psinger/20070917.htm
Still, even before all the details come to light, a few things are clear: 1) It was inevitable. Private military contractors have been involved in all sorts of questionable incidents, since the very start of the Iraq enterprise. U.S. military officers frequently expressed their frustrations with sharing the battlefield with such private forces operating under their own rules and agendas, and worry about the consequences for their own operations. For example, Brigadier General Karl Horst, deputy commander of the US 3rd Infantry Division (responsible for Baghdad area) tellingly put it two years back, These guys run loose in this country and do stupid stuff. There's no authority over them, so you can't come down on them hard when they escalate force. They shoot people, and someone else has to deal with the aftermath."