Washington, May 6 - Khalid Sheikh Mohammed, the alleged mastermind behind the 9/11 terror attack, and four other suspects Saturday appeared at a US military tribunal at Guantanamo Bay.
The arraignment marked the second time the US has tried to prosecute the 9/11 suspects, Xinhua reported.
The five defendants are being held in a classified section under tight security.
Khalid Sheikh Mohammed is accused of obtaining approval and funding from slain Al Qaeda chief Osama bin Laden for the attack, overseeing the entire operation and training the hijackers in all aspects of the operation in Afghanistan and Pakistan.
The overall charges say that the five accused are responsible for the planning and execution of the attacks of Sep 11, 2001, resulting in the death of 2,976 people.
If convicted, they could get the death penalty.
About a dozen family members of 9/11 attack victims and around 60 media representatives were allowed to the naval station to watch the proceedings, the Pentagon said.
The defendants have been provided counsel with specialised knowledge and experience in death penalty cases in a bid to assist them in their defence, it said.
The Pentagon also arranged in the US several viewing sites through closed-circuit TV to show the proceedings to victims' families and members of the media.
In December 2010, the US Congress adopted restrictions on prosecuting Guantanamo prisoners in civilian courts. The five suspects were referred to the Pentagon in April 2011 for trials in military tribunals. (IANS)