Big story in the Washington Post this morning is about the Blackwater controversy in which State Department contract guards are implicated in the September 16 shootings in which sixteen Iraqis were killed. It turns out that potentially incriminating statements made by the guards may not be used against them because State Department investigators effectively immunized them.
Under the rules, the guards are treated as if they were government agents. Government agents can be compelled to answer questions by their agencies on pain of losing their jobs if they demur. But there's a price tag: if they choose to speak under that compulsion, any statements they make may not be used to prosecute them due to the Fifth Amendment's proscription against compulsory self-incrimination.
This is a weighing government always has to make in a situation fraught with political implications: Do we treat it as a diplomatic incident or a crime? Is it more important to find out what happened quickly so it can be explained or to find out what happened methodically so culprits can be prosecuted?
Sometimes, you can do both — but you need to have people who are experienced in this sort of thing. The State Department is perfectly good at getting to the bottom of certain things, but murder (if this was murder) is not in its bailiwick. The FBI, on the other hand, knows how to do this — and knows, in particular, that if you can't crack the case without giving someone immunity, you don't turn around and give everyone immunity. Instead, you try to assess what knowledgeable person has the least culpability and immunize that person — you lose the ability to prosecute him, but the trade-off is you can use his information to prosecute everyone else.
Looks like that may not have happened here. In theory, the FBI may still be able to prosecute, but it would have to do it on the basis of other evidence, and it would have to be able to prove the immunized statements did not advance the case against anyone who gave them. That can be very hard to do.