AP: Blackwater Guards Given Immunity in Deadly Baghdad Shooting

At that point, the Justice Department shifted the investigation to prosecutors in its national security division, sealing the guards' statements and attempting to build a case based on other evidence from a crime scene that was then already two weeks old.

The FBI has re-interviewed some of the Blackwater employees, and one official said Monday that at least several of them have refused to answer questions, citing their constitutional right to avoid self-incrimination. Any statements that the guards give to the FBI could be used to bring criminal charges.

A second official, however, said that not all the guards have cited their Fifth Amendment right against self-incrimination — leaving open the possibility for future charges. The official declined to elaborate.

Prosecutors will have to prove that any evidence they use in bringing charges against Blackwater employees was uncovered without using the guards' statements to State Department investigators. They "have to show we got the information independently," one official said.

Garrity protections generally are given to police or other public law enforcement officers, and were extended to the Blackwater guards because they were working on behalf of the U.S. government, one official said. Experts said it's rare for them to be given to all or even most witnesses — particularly before a suspect is identified.

"You have to be careful," said Michael Horowitz, a former federal prosecutor in Manhattan and senior Justice Department official. "You have to understand early on who your serious subjects are in the investigation, and avoid giving these people the protections."