Army Col. James L. Pohl, the judge, ruled an open hearing would pose a national security risk and ordered Khalid Sheik Mohammed, the self-proclaimed mastermind of the attack, and his co-defendants out of the courtroom Monday, The Miami Herald reported.
Pohl did not say what the risk was.
At issue was a motion labeled AE52, listed as "government consolidated notice regarding ex parte, in camera filing and motion for finding," the Herald said, explaining it was the government secret request for a secret ruling from the judge.
The hearing lasted 36 minutes, said Army Lt. Col. Todd Breasseale, a Pentagon spokesman who couldn't provide specifics besides the motion's title.
Pohl rejected a defense request to include in the hearing the men who, if convicted, could be executed for conspiring in the worst attack on U.S. soil, including 2,976 counts of murder -- Mohammed, 48; Walid bin Attash, 35; Ramzi bin al Shibh, 41; Ammar al Baluchi, 35; and Mustafa al Hawsawi, 45.
James Connell III, Baluchi's military lawyer, argued that the motion's contents weren't classified, the Herald said. Even if the hearing were closed to the public, the defendants should be able to hear arguments because they're held incommunicado at a secret military prison in Guantanamo Bay, Cuba, Connell said.
Prosecutor Joanna Baltes said the U.S. government chose to classify the motion and did not have to justify the reason, the Herald said.
"Despite the government's propaganda, this process is not transparent," said Army Maj. Jason Wright, Mohammed's military defense attorney. "It's seeking to execute men with secret evidence away from the public view."
2014: The Year in Fashion [PHOTOS]
Syrian Al Qaida group executes Lebanese soldier