In releasing the documents, the most comprehensive accounting yet of interrogation methods that were among the Bush administrations most closely guarded secrets, Obama said he wanted to move beyond "a dark and painful chapter in our history."
Past and present CIA officials had unsuccessfully pressed for more parts of the four legal memos to be kept secret, and some critics argued the release would make the United States less safe.
Michael Hayden, who led the CIA under George W. Bush, said CIA officers will now be more timid and allies will be more reluctant to share sensitive intelligence.
"If you want an intelligence service to work for you, they always work on the edge. That's just where they work," Hayden said. Now, he argued, foreign partners will be less likely to cooperate with the CIA because the release shows they "can't keep anything secret."
On the other side, human rights advocates argued that Obama should not have assured the CIA that officers who conducted interrogations would not be prosecuted if they used methods authorized by Bush lawyers in the memos.
Obama disagreed, saying in a statement, "Nothing will be gained by spending our time and energy laying blame for the past."
The Bush administration memos describe the tough interrogation methods used against 28 terror suspects, the fullest and now complete government accounting of the techniques. They range from waterboarding - simulated drowning - to using a plastic neck collar to slam detainees into walls.
Other methods were more psychological than violent. One technique approved but never used involved putting a detainee who had shown a fear of insects into a box filled with caterpillars.
The documents also offer justification for using the tough tactics.
A May 30, 2005, memo says that before the harsher methods were used on top al-Qaida detainee Khalid Sheikh Mohammed, he refused to answer questions about pending plots against the United States.
"Soon, you will know," he told them, according to the memo.
It says the interrogations later extracted details of a plot called the "second wave" to use East Asian operatives to crash a hijacked airliner into a building in Los Angeles.
Terror plots that were disrupted, the memos say, include the alleged effort by Jose Padilla to detonate a "dirty bomb" spreading nuclear radiation.
Even as they exposed new details of the interrogation program, Obama and Attorney General Eric Holder, offered the first definitive assurance that the CIA officials who were involved are in the clear, as long as their actions were in line with the legal advice at the time.
Holder went further, telling the CIA the government would provide free legal representation to its employees in any legal proceeding or congressional investigation related to the program and would repay any financial judgment.
"It would be unfair to prosecute dedicated men and women working to protect America for conduct that was sanctioned in advance by the Justice Department," Holder said.
Obama said in his statement and a separate letter sent directly to CIA employees that the nation must protect their identity "as vigilantly as they protect our security."
Current CIA Director Leon Panetta said in a message to his employees: "CIA responded, as duty requires."
Some parts of the memos were blacked out, and Panetta had pushed for more redactions, according to a government official who declined to be named because he was not authorized to release the information.
The CIA has acknowledged using waterboarding on three high-level terror detainees in 2002 and 2003, with the authorization of the White House and the Justice Department. Hayden said waterboarding has not been used since, but some human rights groups have urged Obama to hold CIA employees accountable for what they, and many Obama officials, say was torture.
The memos produced by the Justice Department's Office of Legal Counsel in 2002 and 2005 were released to meet a court-approved deadline in a lawsuit against the government in New York by the American Civil Liberties Union.
"It's impossible not to be shocked by the contents of these memos," said ACLU lawyer Jameel Jaffer. "The memos should never have been written, but we're pleased the new administration has made them public."
In addition to detailing individual techniques, one memo also specifically authorized a method for combining multiple methods, a practice human rights advocates argue crosses the line into torture even if any individual methods does not.
The methods authorized in the memos include keeping detainees naked, keeping them in painful standing positions and keeping their cells cold for long periods of time. Other techniques include depriving them of solid food and even beating and kicking them.
Sleep deprivation, prolonged shackling and threats to a detainee's family were also used.
Interrogators were told not to allow a prisoner's body temperature or food intake to fall below a certain level, because either could cause permanent damage, said senior administration officials.
The Obama administration last month released nine legal memos from the Bush administration. It probably will release more as the ACLU lawsuit proceeds, the officials said.
The lawsuit has sought to use the Freedom of Information Act to shed light on the treatment of prisoners - though the Bush administration eventually abandoned many of the legal conclusions put forth in the memos and the Obama administration has gone further to actively dismantle much of President Bush's anti-terror program.
Obama has ordered the CIA's secret overseas prisons known as "black sites" closed and has ended "extraordinary renditions" of terrorism suspects to other countries if there is any reason to believe those countries would torture them. He has also restricted CIA questioning to methods and protocols approved for use by the U.S. military until a complete review of the program is conducted.
Also on Thursday, Holder formally revoked every legal opinion or memo issued during Bush's presidency that justified interrogation programs.
The documents have been the subject of a long, fierce debate inside and outside government over how much should be revealed about the previous administration's approach.
In his statement, Obama said he was reassured about the potential national security implications by the fact that much of the information contained had already been widely publicized - including some of it by Bush himself - and by the fact that the program no longer exists as it did.
Withholding the memos, Obama argued, would only serve to deny facts already in the public domain.
"This could contribute to an inaccurate accounting of the past, and fuel erroneous and inflammatory assumptions about actions taken by the United States," the president said.
Those assurances are not likely to inoculate Obama against criticism from conservatives. Last month, former Vice President Dick Cheney said that Obama's decisions to revoke Bush-era terrorist detainee policies will "raise the risk to the American people of another attack."
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