US authorities censored part of a preliminary hearing Monday at a Guantanamo military tribunal that touched on CIA secret prisons where suspected 9/11 plotters say they were tortured.

Reporters watched the proceedings against the five 9/11 suspects at the US naval base in Guantanamo Bay, Cuba behind a thick, sound-proof glass wall and listened to piped-in audio with a 40-second delay.

But when a defense lawyer mentioned the CIA secret sites at Monday's hearing, a red light flashed on and the sound from the courtroom was cut off.

The audio from the proceedings was replaced by white noise, preventing journalists from listening to the courtroom debate over evidence related to the CIA "black sites," where the suspected September 11 attackers were detained and interrogated before being transferred to the Guantanamo facility.

The feed went off after David Nevin, attorney for the self-proclaimed mastermind of the September 11, 2001 attacks, Khalid Sheikh Mohammed, uttered the word "secret."

The defense argues that these "black" sites, whose locations remain secret, should be preserved because they constitute potential evidence that the five were tortured at the prisons.

Defense lawyers say the five accused were subjected to harsh interrogations at the CIA sites, with Mohammed subjected to waterboarding -- or simulated drowning -- dozens of times, as the CIA has acknowledged.

"As a first step we have to know about the building, then about what happened in this building," James Connell, defense lawyer for Pakistani Ali Abd al-Aziz Ali, told a press conference on Sunday.

But military prosecutors argue that any discussion of the CIA's detention program has to remain classified to protect intelligence sources and methods that could prevent a future attack.

At the hearing, the defendants' lawyers each asked the court who had the authority to control the "kill switch" on the audio feed.

"Who has the permission to turn that light on?" asked Nevin.

One of the suspects, Yemeni Walid bin Attash, also objected to the censored hearing, saying there was no "motivating factor" to come to court.

"Our attorneys are bound and we are bound also," he said. "The government doesn't want us to say anything, to do anything."

Judge James Pohl warned that no "external" authority had the power to turn off the video feed without reason.

"If some external body is turning things off at the commission based on their own view of what things ought to be with no reason no explanation, then we're going to have a little meeting about who turns that light on and off," the judge said.

The hearing ended for the day and it appeared questions about censoring references to CIA prisons, as well as the operation of the "kill switch," would be taken up behind closed doors.

The pre-trial hearings resumed after a three-month break for the five suspects, who face the death penalty if convicted of the murder of nearly 3,000 people on September 11, 2001 in the worst ever attack on US soil.

The proceedings are held in a high-tech, maximum-security courtroom specially designed for the proceedings at the remote US base in southeastern Cuba.

The argument over the "kill switch" on the video feed followed the easing of some rules for reporters covering the case.

Spiral notebooks and pens are now permitted at the military commission, officials announced.

In the past, journalists could not bring in their own notebooks or pens.

Reporters, who apply for accreditation through the Pentagon, had previously been provided with a pencil or pen by the military on entering the court and been allowed only loose pieces of paper or notepads with no metal spiral fastener, as part of strict security procedures.

However, cellphones, laptops, cameras and recording devices remain prohibited under the rules.