During this period in time (2007-2008), the Iraqi and United States governments were negotiating the Status of Forces Agreement (SOFA) which outlined the legal status of Coalition forces in Iraq. Iraq was pressing the United States to permit Coalition forces to be tried in Iraqi courts and subject to Sharia Law. They claimed that not being able to do this undermined their sovereignty, and made wild accusations related to the events of September 16th, saying that the guards went into the square unprovoked and slaughtered many Iraqis. It is impossible to know what exactly happened behind those closed doors but its reasonable to assume that the United States government offered our decorated veterans and protectors up as sacrificial lambs. From the DOJ's actions, it is reasonable to conclude that they did not promise an investigation, they promised a conviction, as was reiterated by Vice President Joe Biden years later during a diplomatic visit to Iraq. 

You can find more information about the Status of Forces of Agreement by going to 

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