Defense attorneys insisted they could not proceed with the pretrial hearing in Guantanamo Bay, Cuba, because of the possibility of FBI involvement, and the presiding military judge, Army Colonel James Pohl, opted to end the hearing early on Tuesday and cancel Wednesday's scheduled session.
Defense team member Jim Harrington disclosed at the hearing on Monday that agents had tried to recruit a defense security officer as a spy earlier this month.
Two FBI agents approached the defense officer and asked him "to sign an agreement," said Harrington, an attorney for Ramzi Bin al Shibh, a Yemeni citizen accused of wiring money to the Sept. 11 hijackers.
In addition to cutting the hearing short, the judge said any defense team members who have been approached by government agents will be ordered to disclose it.
The defense on Tuesday said they need to determine whether other defense staff could be working as informants.
"It's really a conflict," Harrington said in court. "We have had our DSO working with us for months and months and months, and we have to question every piece of advice he gave us."
The defense suggested the two FBI agents and the officer be called to testify, but U.S. military attorneys said it was unlikely the agents would disclose information about an ongoing investigation.
In December, Bin al Shibh was repeatedly ordered out of the courtroom for outbursts about "torture" and a "secret CIA prison," and the hearing this week was scheduled to assess his mental competence.
Pohl said the hearing may resume on Thursday.
Held at the Guantanamo Bay U.S. Naval Base in Cuba, the hearing was monitored over a closed-circuit broadcast at the Fort Meade, Maryland, Army base.
The U.S. military has called the defendants "high-value detainees" who played key roles in plotting the September 11 attacks that killed about 3,000 people, destroyed the World Trade Center in New York City and damaged the Pentagon, outside Washington.
The five defendants could be executed if convicted of charges that include mass murder, terrorism and hijacking.
They were captured in 2002 and 2003 and were first charged at Guantanamo in 2008. The tribunals and the charges were revised by the administration of U.S. President Barack Obama, and the defendants were arraigned on the current charges in May 2012.