The Guantanamo Bay war crimes tribunal is back in session in the Sept. 11 case, but without the defendants.

Khalid Sheikh Mohammed and his four co-defendants chose to sit out the second-day of a weeklong pretrial motions hearing. It's not clear why they are boycotting Tuesday's session. A day earlier, defendant Ramzi Binalshibh requested permission to address the court and was denied.

The judge allows them to skip pretrial sessions but says they must attend the trial. Prosecutors have expressed concerns, fearing the men might later argue that they were excluded from court and use that as a basis for an appeal.

The five are charged with nearly 3,000 counts of murder for planning and aiding the Sept. 11, 2001, terrorist attacks. A trial date has not yet been set.