He is known as the 'mastermind' of the September 11, 2001 terrorist attacks, and is the most infamous of all of the detainees held by the US military in Guantanamo Bay, Cuba.

Since his capture in 2003, he has spent years lingering in secret CIA prisons, interrogated under harsh conditions - including waterboarding - before he was transferred to Guantanamo in 2006.

The last time the world saw Khalid Sheikh Mohammed, dubbed KSM by his American captors, was shortly after the CIA and Pakistani authorities took him into custody and released a photo of the heavy- set, disheveled looking prisoner.

The public will get its first view in years of the 'biggest fish' captured so far in the war on terrorism when his trial, and that of four co-defendants, gets underway Thursday at the remote US naval base.

All five suspects face the death penalty if they are convicted on charges they organized the September 11 attacks, and their trial will once again bring the spotlight on the controversial legal process the Bush administration has designed for prosecuting suspects in the war on terrorism.

Mohammed, 54, will be on trial with Ramzi bin al-Shibh, the alleged point of contact between the hijackers and al-Qaeda's leadership; Walid bin Attash, believed to have trained some of the hijackers; Ali Abd al-Aziz Ali, Mohammed's nephew and alleged deputy; and Mustafa Ahmed al-Hawsawi, who allegedly helped the hijackers find flight schools.

They face 169 counts, including conspiracy, murder in violation of the law of war, attacking civilians and civilian objects, and terrorism. Other charges include causing serious bodily injury, destruction of property and providing material support for terrorism. The September 11 attacks killed 2,973 people.

President George W Bush ordered the tribunals under the 2006 Military Commissions Act after the Supreme Court ruled his previous executive order violated US military law, the Constitution and Geneva Conventions.

While critics acknowledge there has been some improvements to the process since the Supreme Court decision, they say the commissions are still flawed and are designed to produce convictions.

'It is a deeply flawed process,' said Jennifer Daskal, a senior counter-terrorism attorney for Human Rights Watch.

Daskal criticized rules that could allow evidence that was obtained from the defendants through abuse and possibly torture, and charged the Bush administration is rushing the process to coincide with the presidential election.

'It's been six-plus years since the first military commissions were first announced and there hasn't been a single trial of anyone to date,' Daskal said.

With the November presidential election on the horizon 'these cases are now being railroaded through,' Daskal said.

Mohammed and the other four men will be allowed to make statements during their appearance Thursday to hear the charges levelled against them. The Pentagon has set a tentative date of September 15 to begin opening arguments.

The Bush administration denies the trial date was set for political reasons, saying they have sought to begin the commissions for years only to be halted by the federal court system.

'I don't think it's any surprise that military tribunals that we've been trying to get going literally for years are now getting going,' State Department deputy spokesman Tom Casey said.

'One of the things that's been made clear is that those, quote, 'high-value detainees' were going to be some of the first that would be put through the military tribunal process,' he added.

The defendants have been appointed military attorneys but are permitted to seek private counsel as well. They will also be allowed to summon witnesses to testify on their behalf and question witnesses testifying against them. They will also be permitted to review the evidence presented against them.

'There will be no secret trials. We will make every effort to make everything open,' said Brigadier General Thomas Hartmann, the principal military advisor for the tribunals.

The Pentagon has invited dozens of journalists to watch the proceedings, the biggest yet since the war on terrorism began and Mohammed, who ranked third in al-Qaeda, was captured.

The Bush administration has said the interrogation of Mohammed has led to the arrests of other terrorists and the thwarting of plots to kill civilians. Bush has acknowledged that he endorsed 'tough' methods for questioning detainees but insists no torture has occurred.

According to the Pentagon, Mohammed admitted in a statement to planning the September 11 attacks and a host of other attacks and unhatched plots, and to personally murdering Wall Street Journal reporter Daniel Pearl in Pakistan, according to a transcript released by the Pentagon in 2007.

'I was responsible for the 9/11 operation from A to Z,' Mohammed purportedly said.