Law enforcement officials say the State Department granted them immunity from prosecution before taking their statements. They can still be prosecuted, but former prosecutor David Laufman said it will be harder to make a case, CBS News national security correspondent David Martin reported.
"It requires the FBI and the department of justice to ensure that any evidence the FBI develops is disconnected from any information that the State Department developed," Laufman said.
The FBI can still interview the guards, but Laufman doubts they will cooperate.
"It is almost certain that they or their attorneys will seek immunity from the Department of Justice before talking to the FBI or appearing before a Grand Jury," he said.
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 employees of North Carolina-based Blackwater 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 protection as investigators from the Bureau of Diplomatic Security sought to find out what happened. The bureau is an arm of the State Department.
The law enforcement and State Department officials agreed to speak only if they could remain anonymous because of the sensitivity of the inquiry into the incident.
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.