State Department promised Blackwater USA bodyguards immunity from prosecution in its investigation of last month's Murder of 17 Iraqi civilia

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As a result, it will likely be months before the United States can — if ever — bring criminal charges in the case that has infuriated the Iraqi government.

"Once you give immunity, you can't take it away," said a senior law enforcement official familiar with the investigation.

A State Department spokesman did not have an immediate comment Monday. Both Justice Department spokesman Dean Boyd and FBI spokesman Rich Kolko declined comment.

FBI agents were returning to Washington late Monday from Baghdad, where they have been trying to collect evidence in the Sept. 16 embassy convoy shooting without using statements from Blackwater employees who were given immunity.

Three senior law enforcement officials said all the Blackwater bodyguards involved — both in the vehicle convoy and in at least two helicopters above — were given the legal protections as investigators from the Bureau of Diplomatic Security sought to find out what happened. The bureau is an arm of the State Department.

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