Files released by the whistleblowing website Wikileaks have revealed that the US believed many of those held at Guantanamo Bay were innocent or only low-level operatives.
The files, published in US and European newspapers, reveal analysis of all 780 people ever held at the facility.
They show that about 220 were classed as dangerous terrorists, but 150 were innocent Afghans and Pakistanis.
Another 380 people were deemed to be low-ranking foot soldiers.
There are now just over 180 detainees at the US naval base in Cuba.
The files also detail alleged plots again British and US targets.
The alleged conspiracies, revealed under interrogation, included claims that a nuclear weapon had been secreted in Europe for detonation should al-Qaeda chief Osama Bin laden be captured.
Other alleged plots include plans to put cyanide into the air conditioning systems of US public buildings and attempts by al-Qaeda to recruit workers at London's Heathrow Airport.
The files - published in the Guardian among other newspapers - showed that US military analysts considered only 220 of those ever detained at Guantanamo to be dangerous extremists.
Another 380 were deemed to be low-ranking foot soldiers who travelled to Afghanistan or were part of the Taliban.
At least another 150 people were innocent Afghans or Pakistanis - including drivers, farmers and chefs - rounded up during frantic intelligence gathering in war zones.
Mistaken identity
These were then detained for years due to mistaken identity or because they were in the wrong place at the wrong time, the memos say.
In many cases, US commanders concluded there was "no reason recorded for transfer".
The Pentagon described the leak as "unfortunate" and described the assessments as snapshots that may now be outdated.
The BBC's Jonny Dymond in Washington says many of the details have been known about in some shape or form before, but not from an official US source.
The 779 documents were part of a massive cache of secret memos leaked to Wikileaks last year.
President Obama pledged in January 2009 to close Guantanamo Bay detention facility within a year.
However, in March this year he announced he was lifting a two-year freeze on new military trials for detainees there.
The White House says Mr Obama remains committed to the eventual closure of Guantanamo Bay.