The arraignment hearing for the five Guantanamo detainees accused of plotting the 2001 terrorist attacks on the United States hit a snag Saturday, as the suspects carried out what appeared to be a silent protest against the proceedings.

The self-proclaimed mastermind of the attacks, Khalid Sheikh Mohammed, and his four co-defendants removed their translation-providing earphones and declined to answer any questions from the military judge. Mohammed's civilian lawyer, David Nevin, said he believed Mohammed was not responding because he was concerned about the fairness of the proceeding.

Saturday's hearing was taking place at the U.S. military base in Guantanamo Bay, Cuba, where the five men have been held for years amid a legal and political battle over how and where to prosecute them.

Defendant Walid bin Attash was held in a restraint chair in the courtroom, before being freed after agreeing to behave. And Ramzi Binalshibh interrupted the session with an outburst asserting he might be killed.

At least one of the defendants also got up and prayed on the courtroom floor as troops providing security kept watch.

The five suspects were being formally charged with killing nearly 3,000 people in the September 11, 2001 attacks in New York, Washington and Shanksville, Pennsylvania. If convicted, they could face execution.

The arraignment is the first step before the beginning of the trial and marks the second time the U.S. has tried to prosecute the September 11 suspects. The legal maneuverings are expected to last for years.

Several family members of the victims of the terrorist attacks were selected by lottery to travel to Cuba to witness Saturday's hearing. Other relatives gathered at military bases inside the United States to watch the proceedings live on closed-circuit television.