A US Army officer representing one of Guantanamo Bay's most notorious prisoners has spoken out against the secretive nature of the Military Commissions system, insisting it risks becoming little more than a "show game" to execute suspects, denying them and the American people the right to a fair trial.

Captain Jason Wright was appointed by the military to represent Khalid Sheikh Mohammed, who is charged along with four others with conspiring and executing the 9/11 attacks. Yet, the officer revealed to The Independent, rafts of vital evidence – including the three and half years his client spent at secret CIA "black" sites – have been deemed classified.

It was only through a Freedom of Information request that redacted files were released showing that Mohammed had been subjected to waterboarding interrogation 183 times, kept awake for seven days straight and had his family's lives threatened.

Defence lawyers are banned from passing documents in confidence to their clients or even discussing certain issues, funding is severely restricted and requests for additional counsel or experts refused. While the accused face the death penalty if convicted, they are likely to be detained indefinitely even if they are acquitted, a situation Capt Wright likened to torture.

"This is not a trial. It is an attempt to legitimise a death threat," he said. "It can never be fair to bring a man to the brink of death and back 183 times with waterboarding and then do it a final 184th time – but this time it will have full state sanction. The bottom line is, torture denies justice. If you believe fundamentally that America is better than that, then this is a struggle for the soul of America.

"I am concerned there has been this show game to try and find the best system in order to try and execute these men. I am deeply concerned by the prospect that theoretically, if anybody is tried and acquitted, the US government has the authority to continue to detain that person indefinitely."

The officer, who previously spent 15 months as a legal adviser in northern Iraq, continued: "I think a lot of Americans are appalled that Guantanamo Bay is still open. No one denies 9/11 was horrific, but there are many people in the Middle East who have felt aggrieved by the past effects of colonialism and neo-colonialism. There has been a cycle of tragedy and trauma and now we can add torture.

"I think it is time for the hyperbole of the war on terror to end, for all sides to pursue both redemption and rediscovery. It is my hope this case will provide an opportunity for this process. But in order to do so, it must be seen as fair and legitimate, not just to America but to the rest of the world." Despite President Obama's promise to close the notorious detention facility in Cuba, 167 inmates – all Muslims – remain of the original 800 who were brought in after thousands of dollars in bounties were offered to those that turned them in. Of those still incarcerated, about half have been cleared but not released.

Capt Wright said that the reformed Military Commissions Act, brought in after Congress defied the President's attempts to have the trials heard within the American legal system, does not abide by the Geneva Convention, allowing evidence to be gained under duress. He said the situation was "still woefully short of the standards of any normal court in the US".

Along with representatives of the four other detainees, he is due to return to court next week to argue that he must be allowed to communicate documents to his client in confidence. Two years ago, the Joint Task Force Commander at Guantanamo Bay ordered that all papers must be passed through a review team.

"All the defence lawyers have refused to abide by this order. Since December, we have been unable to share any confidential documents with our clients," said the judge advocate. "It violates our duty of client confidentiality and our duty of basic due diligence to represent our clients."

The lawyers are banned from discussing certain subjects – even the term "jihad", which is included in the charge sheet. Court hearings have a 40-second delay, so anything deemed confidential can be silenced before the public gallery hears it.

"It has a chilling effect on our ability to perform a basic aspect of the defence investigation. For instance, anything Mr Mohammed conveys verbally to me, whether it relates to his torture, his treatment or even what may have happened on the eve of his capture in Pakistan, remains classified at the highest level," said Capt Wright. "The US government said that because Mr Mohammed was a high-value detainee, he was exposed to classified CIA information and is therefore a holder of classified information. So effectively: 'Because we tortured you, you can't speak'."

An arraignment hearing in May, normally expected to last no more than an hour, stretched out for 13 hours as each one of the defence lawyers unsuccessfully pleaded for the restrictions to be lifted.

Capt Wright, who insists that he must fulfil his obligation to advocate on behalf of his client, added: "There are severe restrictions on our performing our job competently. It is fundamentally unfair, both for the attorney and the client, to have defence counsel subject to control of the US government."

A Department of Defense spokesman said last night: "Every detainee at Guantanamo has had ample opportunity to get the help of lawyers, and there are plentiful examples of robust and functional attorney-client relationships being formed and of zealous, effective representation being provided."

Pointing out that the accused had a right of appeal to a federal civilian court and ultimately to the Supreme Court, he added: "These reformed military commissions... require all of the protections of fairness and justice that are required by our values. The accused is presumed innocent. The prosecution must prove his guilt beyond a reasonable doubt."

Facing death: The accused

Nine years after he was arrested, Khalid Sheikh Mohammed was charged with orchestrating the 9/11 terror attacks. Last April, along with four others – Walid Muhammad Salih Mubarak Bin Attash, Ramzi Binalshibh, Ali Abdul Aziz Ali, and Mustafa Ahmed Adam al-Hawsawi – he was accused of eight offences, including murder, hijacking and terrorism.

The five are "responsible for the planning and execution of the attacks of 11 Sept 2001, in New York, Washington DC, and Shanksville, Pennsylvania, resulting in the killing of 2,976 people," a Department of Defense statement said, adding: "If convicted, the five accused could be sentenced to death." KSM, as he has come to be known, was born in Kuwait in around 1964 but went Pakistan as a teenager before moving to the United States to study mechanical engineering and then on to Afghanistan to fight the Soviet invasion. As well as 9/11, he has been accused of involvement in several terrorist plots, including the Bali nightclub bombings and the Richard Reid shoebomb attempt to blow up an airline. His nephew Ali Abdul Aziz Ali, 34, is a computer technician who was hired in the late 1990s to work for the Modern Electronics Corporation in Dubai. Fluent in English, he described himself as "very open-minded and western-oriented". He is accused of assisting the hijackers with "everyday aspects of life in the West" but insists he was simply helping to boost his income and had no idea who they were.

Ramzi Binalshibh, 40, is a Yemeni who worked as a part-time bank clerk until 1995. He was denied a US visa and travelled to Germany to request political asylum, which was rejected, but he returned to Hamburg with a visa. After the attacks he was accused of being a key facilitator. Walid Muhammad Salih Mubarak Bin Attash, 33, is a Yemeni from a prominent family. He read Islamic Studies in Pakistan before losing his right leg in 1997 fighting in Afghanistan in a battle that killed his brother. The Office of the Director of National Intelligence described him as a "scion of a terrorist family".

Mustafa Ahmed Adam al-Hawsawi, 44, is a Saudi accused of being an organiser and financier of the attacks.

Khalid Sheikh Mohammed: The torture details

Waterboarding involves drowning someone to the brink of death before interrogation.

With its history in the Spanish Inquisition, it involves strapping a detainee by their legs and arms to a board which is tipped head down. A cloth is placed over the mouth and nose while water is poured in at varying heights, giving the sensation of drowning. With a doctor present to prevent death, the detainee is subjected to around minute of pain and panic before being raised up and questioned. While information on his three-and-half-year detention in CIA custody is classified, certain redacted documents obtained through Freedom of Information requests confirm Khalid Sheikh Mohammed was waterboarded 183 times, kept awake for seven and a half days and subjected to authorised enhanced interrogation techniques. Human rights experts insist "enhanced" techniques include sleep deprivation, being subjected to extreme temperatures and having a towel placed around his neck before being thrown against a wall. Mr Mohammed was also subjected to unauthorised treatment like threats to the lives of his family.

Guantanamo Bay: Ten years of detentions for 'unlawful enemy combatants'

January 2002 The Bush administration establishes a detainment and interrogation facility at Guantanamo Naval Base, Cuba, and the first 20 detainees arrive.

March 2003 Khalid Sheikh Mohammed is detained in Pakistan and handed over to the US. He is held by the CIA, accused of being the principal architect behind the 9/11 bombings.

September 2006 The Military Commissions Act of 2006 is passed, which allows certain people to be designated "unlawful enemy combatants", thus making them subject to military commissions. President George W Bush confirms for the first time that the CIA has "high-value detainees" in prisons around the world. Fourteen captives, including Mohammed, are transferred to Guantanamo to face military commissions.

January 2009 President Barack Obama signs an order to suspend proceedings at the Guantanamo military commissions for 120 days and announces that the facility will be shut down.

October 2009 A reformed Military Commissions Act is passed. Critics say it still offers sub-standard justice.

January 2011 The Obama administration abandons attempts to close Guantanamo after Congress blocks the President from bringing accused terrorists before US courts.

March 2011 The President approves the resumption of military trials at Guantanamo Bay.

April 2011 Mohammed and four others are charged with conspiracy, murder, hijacking and terrorism. A team of two military lawyers and one civilian lawyer is appointed.