Throughout the years-long debate about fate of the Guantánamo prison, there's always been one unanswered question: how many detainees are in permanent limbo? That is, how many of them are considered unquestionably too dangerous to release, but just as unquestionably not prosecutable. Now we know:
The Obama administration Monday lifted a veil of secrecy surrounding the status of the detainees at Guantánamo, for the first time publicly naming the four dozen captives it defined as indefinite detainees — men too dangerous to transfer but who cannot be tried in a court of law.
....Administration officials have through the years described a variety of reasons why the men could not face trial: Evidence against some of the indefinite detainees was too tainted by CIA or other interrogation torture or abuse to be admissible in a court; insufficient evidence to prove an individual detainee had committed a crime; or military intelligence opinions that certain captives had undertaken suicide or other type of terrorist training, and had vowed to engage in an attack on release.
The formal classification for these prisoners is "continued detention pursuant to the Authorization for Use of Military Force (2001), as informed by principles of the laws of war," as you can see in the excerpt below.
There are lots of Guantánamo detainees who have no near-term prospect of being prosecuted or released, but still could be if circumstances change. However, even if we handled every single one of them, there's still a hard nut of 46 prisoners with no recourse at all. They will never be tried, and they will never be released.