A federal judge has barred prosecutors from using a crucial witness in the first trial of a former Guantanamo detainee, reigniting a fierce debate over whether the government can successfully prosecute terrorist detainees in civilian court. The trial of Ahmed Khalfan Ghailani, who faces charges in the 1998 bombings of two U.S. embassies in East Africa, has been seen as a test of President Barack Obama's goal of moving many other detainees, like Khalid Shaikh Mohammed, into federal court and, ultimately, closing Guantanamo. In the months since Ghailani was brought to New York from Guantanamo Bay, Cuba, Judge Lewis A. Kaplan of U.S. District Court in Manhattan has rejected defense requests to dismiss the case because of violations of Ghailani's right to a speedy trial and because of accusations he was tortured. But just as the trial was to begin Wednesday, Kaplan ruled that he would not allow the witness to testify. He noted that the government had acknowledged that it had identified and located the witness through interrogation of Ghailani when he was earlier held in a secret overseas jail run by the CIA. His lawyers have said he was tortured there. Kaplan said he was 'acutely aware of the perilous nature of the world in which we live.' 'But the Constitution is the rock upon which our nation rests,' he went on. 'We must follow it not only when it is convenient, but when fear and danger beckon in a different direction. To do less would diminish us and undermine the foundation upon which we stand.' The judge delayed the trial's opening until Tuesday, allowing the government to adjust its strategy or appeal the ruling. Attorney General Eric H. Holder Jr. said the government would be examining the judge's opinion and deciding how to react to it. Speaking in Washington, Holder seemed to play down the ruling's significance. 'We are talking about one ruling, in one case by one judge,' Holder said. 'I think the true test is, ultimately, how are these cases resolved? What happens?' he continued, adding, 'Can they be successfully resolved from the government's perspective?' He said history had shown that hundreds of terrorism and related cases had been resolved through pleas or convictions in civilian court. 'I think it's too early to say that at this point the Ghailani matter is not going to be successful,' Holder said. Ghailani was scheduled to begin trial on charges he conspired in the embassy bombings in Nairobi, Kenya, and Dar es Salaam, Tanzania. The attacks, orchestrated by al-Qaida, killed 224 people. After Ghailani was captured in 2004, he spent nearly five years in CIA custody in so-called black sites and later at the military prison at Guantanamo. Ghailani's lawyers, Peter E. Quijano, Steve Zissou and Michael K. Bachrach, had argued that their client was tortured while in CIA custody, and that any statements he made or evidence derived from those statements, including testimony from the witness whose existence he disclosed, was tainted and inadmissible. After the hearing Wednesday, Quijano praised the ruling, saying, 'This case will be tried upon lawful evidence, not torture, not coercion.' He said the Fifth Amendment had to apply to Ghailani as much as to any other defendant. 'It is the Constitution that won a great victory today,' Quijano said. 'We applaud the court for its courage and support for the law.' Prosecutors say the disputed witness, Hussein Abebe, sold Ghailani the TNT used to blow up the Embassy in Dar es Salaam. They say Abebe agreed voluntarily to testify against Ghailani, and that his decision to cooperate was linked only remotely to the interrogation. Abebe had been characterized by prosecutors as a 'giant witness for the government.' On Friday, a prosecutor, Michael Farbiarz, explained in court that without Abebe's testimony about selling the TNT to Ghailani, 'the government has no way of putting such evidence in front of the jury at all.' But in a three-page order, Kaplan said that 'the government has failed to prove that Abebe's testimony is sufficiently attenuated from Ghailani's coerced statements to permit its receipt in evidence.' The judge said he felt it was appropriate to emphasize that the trial was still proceeding, and that if Ghailani were convicted he faced the possibility of life imprisonment. He added that Ghailani's status of 'enemy combatant' probably would permit his detention as something akin 'to a prisoner of war until hostilities between the United States and al-Qaida and the Taliban end, even if he were found not guilty.' The judge said he would issue a fuller opinion later. (STORY CAN END HERE. OPTIONAL MATERIAL FOLLOWS.) Karen J. Greenberg, executive director of the Center on Law and Security at New York University who observed the hearings on Abebe and advocates trying detainees in civilian court, characterized the order as a 'step forward for the rule of law, and therefore it cannot be a setback for the government.' 'We now know that there are lines when it comes to torture, and this is one,' she said. Kaplan has not ruled on whether Ghailani was mistreated while in detention, but, in his decision, said that he assumed for the purpose of his analysis that Ghailani was coerced during interrogation. The ruling excluding Abebe's testimony will probably make the government's case more complicated to prove and the trial longer. Kaplan did not elaborate on his observation that Ghailani could be detained in the military system if he were acquitted, but the comment spurred some debate among legal experts. Ben Wizner, a senior lawyer with the American Civil Liberties Union, said the conventional wisdom about the Ghailani case was that the justice system works if he is convicted and fails if he is acquitted. 'But that's not how we measure the effectiveness of a criminal justice system,' he said. 'The question is whether the government can present its case and whether the defendant can get a fair trial.' Holder, asked whether the government was considering returning Ghailani to the military system and detaining him indefinitely, replied, 'We intend to proceed with this trial.' 