Private US security contractors accused of shooting dead 17 innocent Iraqis may have been offered partial immunity by the US state department, say reports.
Blackwater guards are currently immune from prosecution in Iraq
Unnamed officials said the offer was unauthorised and could make it much more difficult to prosecute the guards employed by the Blackwater firm.
If confirmed, the revelation may put further strain on US-Iraq relations.
The Iraqi government was furious at the 16 September deaths, and demanded the guards be handed over to face trial.
Blackwater says its staff acted in self-defence.
The Associated Press news agency quotes three unnamed senior law enforcement officials as saying that all the guards involved in the case were given the legal protections as investigators from the Bureau of Diplomatic Security looked into what had happened.
The bureau is the investigative arm of the Department of State.
The guards were offered "limited-use immunity deals", reported the New York Times, which protects them against prosecution for anything they might say in interviews with authorities as long as their statements are true.
A state department spokesman, Sean McCormack, refused to confirm the reports, but said that Secretary of State Condoleezza Rice had tried to ensure "inasmuch as we are able, that if people broke the rules or broke the law, that they're held to account".
The FBI took over the investigation earlier this month, after the justice department realised it would not be able to bring charges based on the guards' statements to the Bureau of Diplomatic Security, reported AP. But some of the Blackwater employees have now reportedly refused to answer further questions from FBI investigators.
Unnamed officials speaking to the New York Times questioned whether Bureau of Diplomatic Security officials had the authority to offer such deals - making the legal validity of the offers unclear.
Nonetheless, if the reports do turn out to be true, they could further complicate the issue of how the guards could be tried.
The contractors are currently granted immunity from prosecution under Iraqi law by Order 17 of the Coalition Provisional Authority - the now-defunct interim body set up by the US-led coalition in the wake of the fall of Saddam Hussein.
Private firms working for the defence department are subject to existing US legislation, but those private firms such as Blackwater working for the state department are not.
Last Wednesday, the head of the BDS Richard Griffin resigned following a state department report which identified an urgent need for tougher oversight of the private firms.
The biggest and best-known of the companies is Blackwater.
The Iraqi government accuses Blackwater of the deaths of the 17 innocent civilians in Baghdad, and says it wants the firm out of the country.
The Iraqi Prime Minister, Nouri Maliki, has described the shooting as a "criminal act" and an inquiry by the Iraqi interior ministry concluded the security guards were "100% guilty".
The head of Blackwater denies the killings were unprovoked, insisting his men had been fired upon.