Full transcripts of his testimony are yet to be publicly released, although his comments were reported in various attending news publications. The pretrial hearings will establish which evidence will be admissible in the eventual trial.
"To protect American lives outweighed the feelings of discomfort of terrorists who voluntarily took up arms against us," he said on the first day of his testimony, which is expected to last two weeks, according to the New York Times. "To me it just seemed like it would be dereliction of my moral responsibilities."
He also said that the C.I.A.'s primary motivation was not to secure prosecutions, but to prevent another "catastrophic attack" in the U.S.
"They were going to go right up to the line of what was legal, put their toes on it and lean forward," he reportedly said.
Five men have been charged as conspirators in the September 11 attacks on the World Trade Center, and all were present at the tribunal. Among them was Khalid Sheikh Mohammed, the suspected mastermind behind the attack, who confessed to his role in 2008.
Mitchell and Jessen were called as witnesses by defense lawyers for the suspected attackers, who say that the accused 9/11 conspirators were tortured. Serious doubts remain as to whether evidence and confessions established during the "enhanced interrogation" process would be admissible in open court.
The psychologists have long claimed that their work helped saved lives by gathering information to thwart terror attacks, but the 2014 Senate Select Committee Report on the CIA’s detention program said that their methods, adopted by the CIA, did not reap effective results.
That report concluded that enhanced interrogation techniques used in this period, such as waterboarding, sleep deprivation over a "significant repetition of days," and forcing detainees to strip naked, were "not an effective means of acquiring intelligence."
The Senate’s report said that the CIA’s justifications relied on "inaccurate claims of their effectiveness" and the "interrogations of CIA detainees were brutal and far worse" than lawmakers previously thought.
While in the employ of the CIA, Mitchell and Jensen earned as much as $1,000 a day. In 2005, Jessen and Mitchell formed a company to conduct their work for the CIA. Thereafter, "the CIA outsourced virtually all aspects of the [Detention and Interrogation] program," with the contractors receiving $81 million from the CIA by the time their contract was terminated in 2009, according to the Senate report.
"Their appearance is important to bringing out the facts of how the interrogation program was set up and conducted," Brig. Gen. Stephen Xenakis, a psychiatrist who has been advising attorneys for Gitmo inmates and is attending the hearings, told ABC News. "They did not 'pioneer' any new or effective tactics. They applied tactics that had been known for decades and regarded as harmful."
On the first day of his testimony, "Mitchell identified [the then-top clandestine service official in the CIA] Jose Rodriguez as pushing the program," Xenakis said.
"The victims' families are denied justice because the history of torture has made it nearly impossible to prosecute," he added.
Mitchell said that C.I.A. interrogators took the techniques he recommended, such as waterboarding, too far, according to The Guardian.
"When people are left to make up coercive measures, it tends to escalate over time," he reportedly said. "They dehumanize the detainees. They think they are justified in using a higher level of pressure. They think: If a little is good, a lot is better."
Mitchell and Jessen were heavily criticized for "leaving a stain on the discipline of psychology" by the American Psychological Association (APA) for their roles in the CIA program. Jessen was never a member of the organization, and although Mitchell resigned his membership in 2006, the APA said his conduct would have been enough to expel him if he had remained a member.
The war crimes trial of the 9/11 defendants is scheduled to begin in early 2021.