I honestly don't know why they let the FBI into gitmo:
FBI documents claims of abuse at Guantanamo (BBC story)
If anyone's going to thoroughly document anything that could potentially burn them, it's the FBI. Some of the claims seem more plausible than others, at least based on my read of American behavior. That said, the FBI reports document prisoner claims without any additional attempt to corroborate, so there's no way to do more than have a feeling about plausibility.
If these claims (1) come out (as they now have) and (2) are corroborated, the Bureau can officially declare, "We told you so." If not, then their organization has not ruffled any feathers with the hierarchy at Guantanamo.
I suspect the truth is somewhere between the extremes of "all claims are true" and "all claims are false."
So was the unnamed source for the Newsweek report failing to stick to his guns or making up stories in the first place?