Washington (AFP) - Two psychologists who helped design the CIA's post-9/11 interrogation program settled a lawsuit Thursday by detainees alleging they were illegally tortured.

The secret settlement in the suit, brought on behalf of two living ex-detainees and one who died of hypothermia after brutal questioning in US custody, avoided what would have been the first public trial of the Central Intelligence Agency's use of torture on suspected Al-Qaeda members.

But it also allowed the two psychologists who supplied the CIA with "coercive" interrogation techniques, James Mitchell and Bruce Jessen, to maintain that they personally had nothing to do with the use of waterboarding, extreme stress positions and beatings on detainees.

"Neither Dr. Mitchell nor I knew about, condoned, participated in, or sanctioned the unauthorized actions that formed the basis for this lawsuit," Jessen said in a mutually agreed statement attached to the settlement.

"We served our country at a time when freedom and safety hung in the balance."

The American Civil Liberties Union brought the suit in 2015 against Mitchell and Jessen, who were recruited by the CIA in 2002 to design and help conduct interrogations of war-on-terror suspects captured in Afghanistan and elsewhere.

- CIA paid $80mn for their expertise -

The two were paid around $80 million for their work, which included helping interrogate Khalid Sheikh Mohammed, the mastermind of the September 11, 2001 attacks by Al-Qaeda, and Abu Zubaydah, another top Qaeda suspect.

The suit accused Mitchell and Jessen of responsibility for the CIA's use of torture methods on the three detainees, Tanzanian Suleiman Abdullah Salim, Libyan Mohamed Ahmed Ben Soud, and Gul Rahman of Afghanistan.

Rahman died of hypothermia in a CIA prison cell in November 2002, after what the ACLU says were two weeks of brutal torture.

The ACLU had sought to pin responsibility in part on the psychologists, as well as gain a significant financial award for the men and Rahman's family. The trial had been set to begin on September 5.

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The ACLU and lawyers for the defendants declined to give any details of the settlement, including whether any money would be paid as part of it.

In the settlement statement, the plaintiffs stood by their allegations that Mitchell and Jansen had responsibility for the extreme interrogation methods used on them.

However, they said afterward, "We brought this case seeking accountability and to help ensure that no one else has to endure torture and abuse, and we feel that we have achieved our goals."

"We were able to tell the world about horrific torture, the CIA had to release secret records, and the psychologists and high-level CIA officials were forced to answer our lawyers' questions," they said.

James Smith, the main lawyer for Mitchell and Jensen, said the treatment of the three, while regrettable, was not the fault of his clients.

"If this case had gone forward, the facts would have borne out that while the plaintiffs suffered mistreatment by some of their captors, none of that mistreatment was conducted, condoned or caused by Drs. Mitchell and Jessen," Smith said.