The judge presiding over the first civilian trial of a Guantanamo detainee has ruled that the US government cannot coerce a suspect to provide information for intelligence purposes, then use the evidence in criminal proceedings.
The landmark ruling in New York is likely to set a legal precedent for the handling of evidence gathered from interrogations of other detainees subjected to CIA interrogation after the September 11 2001 attacks.
It also branded a man the government once said was its most important witness as a liar.
In the redacted ruling, US district judge Lewis Kaplan explained his reasons for deciding last week to block the witness, Hussein Abebe, from the trial of Ahmed Khalfan Ghailani.
The Obama administration has said it hopes to bring professed 9/11 mastermind Khalid Sheikh Mohammed and several others to civilian courts.
Judge Kaplan said prosecutors failed to show Ghailani's rough CIA interrogation at a secret camp overseas played no role in getting the witness to co-operate. "If the government is going to coerce a detainee to provide information to our intelligence agencies, it may not use that evidence - or fruits of that evidence that are tied as closely related to the coerced statements as Abebe's testimony would be here - to prosecute the detainee for a criminal offence," the judge said.
But the judge stopped short of expressing an opinion on the constitutionality of the fact that the "CIA, acting upon the highest authority, used coercive methods to gain intelligence", saying the issue was not before him.
Ghailani is charged with conspiracy in the August 1998 bombings of two US embassies in Africa. The attacks killed 224 people, including a dozen Americans.
Prosecutors had argued that Abebe, a Tanzanian miner, would tell jurors that he supplied Ghailani with substantial quantities of explosives, detonation cord and electric detonators, information the government said was "essential to any full and fair understanding of the defendant's role in the embassy bombings".
The defence claimed prosecutors learned about Abebe only after the CIA tortured Ghailani after his 2004 arrest in Pakistan. Ghailani was transferred to Guantanamo Bay in Cuba in 2006 and was brought to New York for trial last year.