An apparent FBI investigation has at least temporarily shut down the effort to try five Guantanamo Bay prisoners by military commission for the Sept. 11 terror attack.

The U.S. government brought about 200 people to the U.S. base in Cuba for a hearing into whether one defendant is mentally competent to stand trial in the death penalty case.

But that plan has been dashed by the revelation that the FBI is investigating how the self-proclaimed mastermind of the attack was able to send two letters and an essay out of the prison without passing through a security review.

The judge began an inquiry Tuesday into the FBI investigation and how it may affect the case over despite a prosecution request to go ahead with the mental competency hearing.