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 says it will be harder to make a case, CBS News national security correspondent David Martin reports.
"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 was brought into the case more than two weeks after the September 16 shooting, which left 17 Iraqi civilians dead. By then the crime scene was once again a busy Baghdad traffic circle and most of the physical evidence had been removed.
"It's always a problem when investigators are getting to the scene of a crime late in a case involving alleged acts of physical violence. Witness' memories are faded. Physical evidence has degraded," he said.
The FBI can still interview the guards but Laufman doubts they will cooperate.
"It is almost certain that they of their attornies will seek immunity from the Department of Justice before talking to the FBI or appearing before a Grand Jury," he said.
The FBI investigators are on their way back from Iraq but because of the complications caused by the grant of immunity any prosecutions could still be months away.