Two weeks ago, a pair of F.B.I. agents appeared unannounced at the door of a member of the defense team for one of the men accused of plotting the 9/11 terrorist attacks. As a contractor working with the defense team at Guantnamo Bay, Cuba, the man was bound by the same confidentiality rules as a lawyer. But the agents wanted to talk. They asked questions, lawyers say, about the legal teams for Ramzi bin al-Shibh, Khalid Shaikh Mohammed and other accused terrorists who will eventually stand trial before a military tribunal at Guantnamo. Before they left, the agents asked the contractor to sign an agreement promising not to tell anyone about the conversation. With that signature, Mr. bin al-Shibh's lawyers say, the government turned a member of their team into an F.B.I. informant. The F.B.I.'s inquiry became the focus of the pretrial hearings at Guantnamo this week, after the contractor disclosed it to the defense team. It was a reminder that, no matter how much the proceedings at the island military prison resemble a familiar American trial, the invisible hand of the United States government is at work there in ways unlike anything seen in typical courtrooms. It's a courtroom with three benches," said Eugene R. Fidell, who teaches military justice at Yale Law School. "There's one person pretending to be the judge, and two other agencies behind the scenes exerting at least as much influence." Thirteen years after 9/11, nobody has been convicted in connection with the attacks.