GUANTANAMO BAY NAVAL BASE, CUBA (AP) — The effort to try five Guantanamo Bay prisoners by Military Commission for the Sept. 11 terror attack has been shut down at least for now by an apparent FBI investigation.

The FBI is looking into 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 in the case has begun an inquiry into how that investigation may affect the case.