There's a critical unanswered question about the torture investigation -- or "preliminary review" announced yesterday by Attorney General Eric Holder. And the Justice Department doesn't seem eager to clear it up.

Who, exactly, is to be investigated?

In his statement announcing the appointment of a special prosecutor, John Durham, Holder said that the probe would consider "whether federal laws were violated in connection with the interrogation of specific detainees at overseas locations."

He also made clear that people "who acted in good faith and within the scope of the legal guidance given by the Office of Legal Counsel regarding the interrogation of detainees" would not be prosecuted.

So that would seem to leave only those CIA personnel -- both employees and contractors -- who overstepped OLC's legal advice.

But what about the OLC lawyers themselves, like John Yoo and Jay Bybee, who provided that advice? Almost every independent expert agrees that Yoo et al. approved activities that went way beyond what's legal. And what about the senior Bush officials, including Dick Cheney, who drew up the policies in the first place?

Holder's reference to "the interrogation of specific detainees at overseas locations" suggests that those Bushies will escape Durham's scrutiny. On the other hand, nothing in Holder's statement explicitly rules out looking at them if the investigation leads that way.

When TPMmuckraker asked the Justice Department for clarification, a spokeswoman evaded the question, telling us: "The [Office of Professional Responsibility] report that has been much discussed covers the OLC lawyers -- at least in their role as attorneys and professional conduct." She did not respond to a follow up asking specifically about the mandate for Durham's investigation.

It's not hard to understand why the Justice Department might be evasive on this. On the one hand, it can't allow Republicans to portray the probe as a partisan witch-hunt designed merely to go after political opponents. But that the same time, it can't explicitly announce that, for political reasons, Durham's hands will be tied from the start, in terms of who he can and can't investigate. Hence the calculated ambiguity.

Still, we're not the only ones who are concerned about the issue. A number of leading Democrats -- including House Judiciary chair John Conyers and Senator Russ Feingold -- yesterday issued statements that applauded Holder for opening the review, but urged him not to limit its scope only to low-level CIA personnel.

Who the investigation looks at is a key question. It seems likely that DOJ will have to offer a bit more on this eventually.