Washington ? The State Department promised Blackwater USA bodyguards immunity from prosecution in its investigation of last month’s deadly shooting of 17 Iraqi civilians, The Associated Press has learned.

The immunity deal has delayed a criminal inquiry into the Sept. 16 killings and could undermine any effort to prosecute security contractors for their role in the incident 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.

State Department officials declined to confirm or deny that immunity had been granted. One official – who refused to be quoted by name- said: “If, in fact, such a decision was made, it was done without any input or authorization from any senior State Department official in Washington.”

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.

The investigative misstep comes in the wake of already-strained relations between the United States and Iraq, which is demanding the right to launch its own prosecution of the Blackwater bodyguards.

Blackwater spokeswoman Anne Tyrrell declined comment about the U.S. investigation. Based in Moyock, N.C., Blackwater USA is the largest private security firm protecting U.S. diplomats in Iraq.

The company has said its Sept. 16 convoy was under attack before it opened fire in west Baghdad’s Nisoor Square, killing 17 Iraqis. A follow-up investigation by the Iraqi government, however, concluded that Blackwater’s men were unprovoked. No witnesses have been found to contradict that finding.

An initial incident report by U.S. Central Command, which oversees military operations in Iraq, also indicated “no enemy activity involved” in the Sept. 16 incident. The report says Blackwater guards were traveling against the flow of traffic through a traffic circle when they “engaged five civilian vehicles with small arms fire” at a distance of 50 meters.

The FBI took over the case early this month, officials said, after prosecutors in the Justice Department’s criminal division realized it could not bring charges against Blackwater guards based on their statements to the Diplomatic Security investigators.