The two psychologists who designed the US “enhanced interrogation” programme that included waterboarding and other forms of torture are due to give evidence in open court for the first time this week.

James Mitchell and Bruce Jessen will answer questions at a pre-trial hearing on the 9/11 attacks before a military tribunal in Guantánamo Bay.

Lawyers for the defendants, who are among 40 detainees being held at prison camp on the island of Cuba, say it will be a unique opportunity to hold to account those responsible for approving and carrying out the use of torture, and to demonstrate that both the CIA and FBI were complicit in torture, with significant implications for any future trial of suspected 9/11 plotters.

Mitchell and Jessen were former air force psychologists who were tasked by the CIA in 2002 to establish a programme of severe interrogation techniques. They were paid $1,800 a day and in 2005 they set up a private company, which provided most of the interrogators and most of the security staff at the “black sites”, secret detention facilities. The company was paid $81m for its services before its contract was terminated in 2009.

“The perverse ‘work’ of these psychologists has dramatically set back the global fight against torture. The interrogation methods they championed have had a rippling effect around the world,” said Julia Hall, a human rights lawyer with Amnesty International who is attending the hearings.

The American Psychological Association has disowned Mitchell and Jessen for “violating the ethics of their profession and leaving a stain on the discipline of psychology”.

But both men have insisted they did nothing wrong, arguing they were asked to do things that were declared legal by the George W Bush administration, and that they had to prevent the worst excesses of other interrogators.

Defence lawyers and human rights advocates hope Mitchell and Jessen will cast more light on the scale of the torture programme, the culpability of senior officials and the role of the FBI, which has hitherto presented itself as uninvolved.

“The main points that I will be asking the witnesses about are the deep involvement of the FBI in the rendition detention and interrogation program, the huge bureaucracy that was necessary to support the use of coercive pressure as an interrogation tactic, and the elements of the CIA’s programme that didn’t involve Dr Mitchell and Dr Jessen at all,” said James Connell, lawyer for one of the defendants, Ammar al-Baluchi, who will lead the questioning of Mitchell this week. “There are many other people who are involved.”

Alka Pradhan, another Baluchi lawyer who will take the lead in questioning Jessen said the coming session was “probably one of the most consequential hearings we have had yet” in eight years of pre-trial hearings.

At the core of the prosecution case are a set of statements made by the defendants in 2007 to FBI investigators. The government contends that this was a “clean team” of investigators who had nothing to do with torture. The statements were therefore untainted and admissible in court.

Pradhan said the Mitchell and Jessen testimony would add to already considerable evidence that the FBI also had dirty hands.

“They knew that they needed ‘clean interrogations’ in order to be able to prosecute and eventually execute,” she said. The defence lawyers say there were FBI agents at the black sites during enhanced interrogations and that the FBI submitted questions for the interrogators to use, knowing torture was being used.

This week’s hearing has been complicated by a new set of secrecy rules introduced by the prosecution on Thursday, that prevents defence lawyers from citing references to the interrogation programme in published books, even though they had been cleared for publication by the CIA.

On the other hand, defence teams will be allowed to cite more details of dates, places and techniques used on detainees. The CIA has acknowledged that at least 39 prisoners were subjected to the enhanced interrogation techniques that included slamming detainees into a wall, confining them in a 21x30in (53x76cm) box, and waterboarding, the simulation of drowning by covering a prisoner’s face with cloth and pouring water on it. The UN and human rights groups have declared waterboarding and some of the other techniques to be torture.

Defence and prosecution lawyers are due to argue over classification and other procedural issues in closed session at the military commission built on the Guantánamo naval base near the detention camp.

Then the presiding judge, Col Shane Cohen, is expected to hold open sessions for at least some of the next two weeks.

Mitchell and Jessen agreed an out of court settlement in a civil suit brought by the American Council on Civil Liberties on behalf of Suleiman Abdullah Salim and Mohamed Ben Soud, who were released without charges after being tortured, and the family of Gul Rahman, who died under interrogation in 2002 in one of the CIA’s secret detention facilities around the world, known as “black sites”.

Rahman died after being left shackled overnight naked from the waist down in a freezing concrete cell. Mitchell and Jessen said they had complained about Rahman’s treatment but their warnings were ignored by senior CIA officials.

The psychologists said their interrogation programme had been designed to induce a “learned helplessness” in detainees that would make them more compliant. Mitchell wrote a memoir, Enhanced Interrogation, in which he claimed that the enhanced interrogation techniques helped foil al-Qaida attacks on the US.

“I concluded that it would be immoral and unethical to ignore my obligation to use what I knew to defend our citizens and our way of life against enemies who themselves had initiated the conflict and whose stated goal was to destroy us,” Mitchell wrote.

His claims over the programme’s success is disputed by a report by the Senate intelligence committee, which found that the enhanced interrogations produced no actionable intelligence.