The findings, detailed in a hearing Tuesday, brought rebukes of the Pentagon effort from Democrats and Republicans alike.
"The guidance (administration lawyers) provided will go down in history as some of the most irresponsible and shortsighted legal analysis ever provided to our nation's military and intelligence communities," said Sen. Lindsey Graham, R-S.C., an Air Force Reserve colonel who teaches military law for the service.
The hearing is the Senate Armed Services Committee's first look at the origins of harsh interrogation methods and how policy decisions were vetted across the Defense Department. Its review fits into a broader picture of the government's handling of detainees, which includes FBI and CIA interrogations in secret prisons.
The panel is expected to hold further hearings on the matter and release a final report by the end of the year.
Among its initial findings is that senior Pentagon lawyers, including the office of general counsel William "Jim" Haynes, sought information as early as July 2002 regarding a military program that trained U.S. troops how to survive enemy interrogations and deny foes valuable intelligence.
Much of the training program, known as "Survival, Evasion, Resistance and Escape," or SERE, is based on experiences of American prisoners of war in previous conflicts, including those in Korea and Vietnam.
In response, SERE officials provided Haynes' office a list of tactics that included sensory deprivation, sleep disruption and stress positions.
Haynes, who resigned his post in February, testified that he remembers receiving the information, but that he did not recall requesting it personally.
Several of those techniques, including stress positions, were later approved by Defense Secretary Donald H. Rumsfeld in a December 2002 memo for use at Guantanamo Bay. Rumsfeld and Haynes agreed to the methods, despite objections by military service lawyers that they might be illegal.
"Whatever interrogation techniques we adopt will eventually become public knowledge," wrote Col. John Ley of the Army's Judge Advocate General office in November 2002. "If we mistreat detainees, we will quickly lose the (moral) high ground and public support will erode."
Haynes said he too had misgivings, but that he was unaware of the legal objections in the military services. He said he was doing the best he could to help prevent another major terrorist attack.
"There was a limited amount of time and a high degree of urgency," Haynes said of his decision to cut short at one point a department-wide review of the legality of the interrogation methods.
Sen. Carl Levin, D-Mich., chairman of the Armed Services Committee, said Rumsfeld's endorsement paved the way for abuses to occur in Iraq and Afghanistan and makes U.S. troops more likely to someday be tortured if captured by the enemy.
"If we use those same techniques offensively against detainees, it says to the world that they have America's stamp of approval," said Levin.
The committee also released previously secret and privately held documents on Tuesday. According to minutes from an October 2002 meeting, a top military lawyer at Guantanamo said prisoners were exposed to previously forbidden techniques, such as sleep deprivation, but that such treatment was hidden from the International Committee of the Red Cross.
"Officially it is not happening," Lt. Col. Diane Beaver said in the meeting. "It is not being reported officially. The ICRC is a serious concern. They will be in and out, scrutinizing our operations, unless they are displeased and decide to protest and leave. This would draw a lot of negative attention."
A senior CIA lawyer at the meeting, John Fredman, explained that whether harsh interrogation amounted to torture "is a matter of perception." The only sure test for torture is if the detainee died.
"If the detainees dies, you're doing it wrong," Fredman said.
Beaver wrote a now-infamous Oct. 11, 2002, memo that determined abusive methods could be used against detainees at Guantanamo Bay prison because they were not considered prisoners of war. Her proposed methods included extended isolation, 20-hour interrogations, death threats and waterboarding.
On Tuesday, Beaver told the committee that she was "shocked" that her memo became the primary justification for Rumsfeld's approval to use harsher methods.
She had asked her superiors for input because those working at Guantanamo and engaged in the interrogation program "don't always have the best perspective."
White House spokesman Tony Fratto said the administration does not review every legal opinion, but that its position has been "to deal with these detainees humanely" and "get the information from them that we can to protect this country."
Notably absent from the hearing Tuesday was the Senate's biggest champion of detainee rights and the top Republican on the committee, Sen. John McCain, R-Ariz. A former prisoner of war, McCain has become less visible on the issue of detainee treatment since becoming a presidential candidate.
McCain was in San Antonio on Tuesday giving a speech on energy and attending campaign fundraisers.