FORT MEADE, Md. (AP) — An architect of the brutal CIA interrogation and detention program developed after the Sept. 11 attacks defended the agency and its practices on Tuesday as those techniques become the focus of an effort to dismiss key evidence against five men charged in the terrorist plot.

James Mitchell spent the first day of what is expected to be at least a week of questioning by defense teams at the U.S. base at Guantanamo Bay, Cuba, providing details about the CIA’s interrogation program as well as what he said was the “context” necessary to understand it.

The CIA was the “tip of the spear” in the months after the Sept. 11, 2001, terrorist attacks and was urgently trying to gather vital intelligence using techniques that had been authorized by the U.S. government, the retired Air Force psychologist told the court.

“We were trying to save American lives,” Mitchell said.

Mitchell is facing questions now because lawyers for the five men accused of planning and providing logistical support for the Sept. 11 attacks are seeking to prevent the government from using statements the defendants gave to the FBI as evidence against them in a war crimes trial scheduled to start next January at the U.S. base in Cuba.

The testimony in Guantanamo is an important milestone in the Sept. 11 war crimes proceedings, which have been bogged down in the pretrial phase since the May 2012 arraignment.

The five defendants, who include the self-proclaimed mastermind of the Sept. 11 hijacking plot, were subjected to waterboarding and other methods now widely regarded as torture. Mitchell, who helped develop the program with another private contractor and others, insisted the CIA feared “another catastrophic attack,” possibly involving nuclear weapons, and was trying to stop it.

“My sole focus was stopping the next attack,” he said.

Mitchell agreed to come to Guantanamo to testify without a subpoena to give his version of events, which he also detailed in a book, called “Enhanced Interrogation,” that he co-wrote with a CIA spokesman.

“I’m happy to talk about my role in the program and what the program did,” he told the court.

At times, however, he appeared to bristle at the questioning. When defense lawyer James Connell thanked him for coming to court, he replied, “I did it for the victims and families not for you.”