70,000 US Mercenaries in Iraq

The kidnapping of four American security contractors earlier this month in Iraq revived allegations that U.S. private security companies are involved in the current bloodshed in Iraq. Iraq war is not just fought by occupation armies and resistance fighters- private firms, consisting of gun-wielding ex-soldiers, are also involved.
Could some of the Pentagon’s hired Mercenaries be the real perpetrators of the daily bombings and assassinations of Sunnis and Shias in Iraq?

Are the 70,000 Pentagon Mercs in Iraq killing Shias, Sunnis?