The defense lawyers’ larger strategy has been to argue that because the United States government tortured their clients, it should not be permitted to execute them as mitigation — an issue that would come to a head during the sentencing phase after a trial and conviction. It is not clear how the reasoning behind Colonel Pohl’s decision to suppress the F.B.I. statements, if it stands, will affect that question.
Asked for a comment on behalf of the prosecution, Cmdr. Sarah Higgins of the Navy, a spokeswoman for the Office of Military Commissions, said officials would “let the rulings speak for themselves when they post” on the tribunal’s website.
But James Connell, a lawyer for another Sept. 11 defendant, Ammar al-Baluchi, hailed the ruling.
“Witnesses are the foundation of the American criminal justice system,” Mr. Connell said. “If the government prohibits the defense from investigating witnesses, the proceeding becomes more like a play than a trial.”
Still, an appeal by prosecutors seemed likely.
The chief prosecutor, Brig. Gen. Mark Martins of the Army, and his team had argued that the law permitted some concessions to national security imperatives in such a case, while playing up how much information the government had turned over in discovery about the black-site program, including summaries about what agency employees and contractors who served as guards, doctors and interrogators at the prisons saw and did. Most of those people are described using code names.
After providing that information, the prosecution told defense lawyers last year that they were not allowed to directly approach people they believed might be such witnesses or to travel to countries they believed might have hosted prisons to ask questions. Rather, the defense has to go through the government to request interviews, prosecutors said, and the government in turn approaches witnesses and asks if they want to talk to defense lawyers while also telling them what they can and cannot discuss.
In practice, Colonel Pohl wrote, only a few of the dozens of witnesses the defense asked to speak with agreed to talk under those conditions.
The Sept. 11 case defendants offered to plead guilty in late 2008, but after the Obama administration took office, it tried to move the case to civilian court. After Congress blocked that option, the defendants were rearraigned before a Guantánamo tribunal in 2012 and pleaded not guilty. Years of pretrial hearings and delays have ensued.