"Once you give immunity, you can't take it away," a senior law enforcement official familiar with the investigation told the Associated Press on Monday.

US state department officials would not confirm or deny that immunity had been granted.
 
But one official, who refused to be quoted by name, told Associated Press: "If in fact such a decision was made, it was done without any input or authorisation from any senior state department official in Washington."

Three senior law-enforcement officials said that all the bodyguards involved in the incident, both in the vehicle convoy and in at least two helicopters above, were given immunity. 

'No enemy activity'

The company has said that the convoy was under attack before it opened fire in west Baghdad's al-Nisoor Square on September 16.
 
A follow-up investigation by the Iraqi government, however, concluded that Blackwater's actions were unprovoked.
 

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An initial incident report by US Central Command, which oversees military operations in Iraq, also indicated "no enemy activity involved" in the incident.

The report says Blackwater guards were travelling against the flow of traffic through a roundabout when they "engaged five civilian vehicles with small arms fire" at a distance of 50 metres.

The New York Times said on Monday that the immunity offer was made by agents from the Bureau of Diplomatic Security, the investigative arm of the agency, although they do not have the authority to do so.

The report said Blackwater guards had been offered what officials described as limited-use immunity, meaning they would not be prosecuted over statements as long as they were true.

FBI investigation

The deal has reportedly delayed the criminal investigation into the shooting.

FBI agents, who took over the investigation from the state department earlier this month, have reportedly been trying to collect evidence in Baghdad without using statements from the employees who were given immunity.

Blackwater has about 1,000 employees in Iraq
protecting diplomats and officials  [File: EPA]
Officials told the Associated Press that the FBI took over the case after prosecutors in the justice department realised they could not bring charges against Blackwater guards based on their interviews with state department investigators.

The FBI has reinterviewed some of the Blackwater employees, but one official said on Monday that some of them had refused to answer questions, citing their constitutional right to avoid self-incrimination.

Any new statements that the guards give to the FBI could be used to bring criminal charges.

Strained relations

North Carolina-based Blackwater has about 1,000 employees in Iraq. It is the largest of three private security firm protecting US diplomats in the country.

The immunity offer is likely to further strain relations between Washington and Baghdad, which is demanding the right to launch its own prosecution of the Blackwater bodyguards.

Nuri al-Maliki, the Iraqi prime minister, has said his cabinet is drafting legislation that would force the state department to replace Blackwater with another security company.