Seeking to move beyond what he calls a "a dark and painful chapter in our history," President Barack Obama said Thursday that CIA officials who used harsh interrogation tactics during the Bush administration will not be prosecuted.

The government also released four memos long held secret by the Bush administration in which its lawyers approved in extensive and often graphic detail the tough interrogation methods used against 28 terror suspects, the fullest and now complete government accounting of the techniques. The rough tactics range from waterboarding _ simulated drowning _ to using a plastic neck collar to slam detainees into walls.

Even as they exposed new details of the interrogation program, Obama and Attorney General Eric Holder, offered the first definitive assurance that those CIA officials are in the clear, as long as their actions were in line with the legal advice at the time.

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."

"We have been through a dark and painful chapter in our history," the president said. "But at a time of great challenges and disturbing disunity, nothing will be gained by spending our time and energy laying blame for the past."

Holder told the CIA that the government would provide free legal representation to CIA 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.

As current CIA Director Leon Panetta put it in a message to employees: "CIA responded, as duty requires."

The CIA has acknowledged using waterboarding, a form of simulated drowning, on three high-level terror detainees in 2002 and 2003, with the permission of the White House and the Justice Department. Former CIA Director Michael 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.

Stacy Sullivan of Human Rights Watch said Obama is "dead wrong" to say that nothing would be gained by examining the past for criminal acts.

"Prosecuting violations of the law is not about laying blame for the past, it's about ensuring that those crimes don't happen again," she said.

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.

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 them include keeping detainees naked for long periods, keeping them in a painful standing position for long periods, and depriving them of solid food. Other tactics included using a plastic neck collar to slam detainees into walls, keeping the detainee's cell cold for long periods, and beating and kicking the detainee. 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 caloric intake to fall below a certain level, because either could cause permanent damage, said senior administration officials. They discussed the memos on condition of anonymity to more fully describe the president's decision-making process.

The Obama administration last month released nine legal memos. It probably will release more as the lawsuit proceeds, the officials said.

The ACLU suit has sought to use the Freedom of Information Act to shed light on the treatment of prisoners _ even though the Bush administration eventually abandoned many of the legal conclusions and the Obama administration has gone further to actively dismantle much of President George W. Bush's anti-terror program.

Obama has ordered the CIA's secret overseas prisons known as "black sites" closed and ended so-called "extraordinary renditions" of terrorism suspects if there is any reason to believe the third country would torture them. He has also restricted CIA questioners to only those interrogation methods and protocols approved for use by the U.S. military until a complete review of the program is conducted.

Also 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 in and outside government over how much officials should say.

The Bush administration held the view that the president had the authority to claim broad powers that could not be checked by Congress or the courts in order to keep Americans safe. Obama and Holder, among others, have said that the use of such unchecked powers has actually made Americans less safe, by increasing anti-U.S. sentiment, endangering American troops when captured and handing terrorists a recruiting tool.

"Enlisting our values in the protection of our people makes us stronger and more secure," Obama said in his statement.

Even so, the officials described the president's process of deciding how much to release in response to the suit as very difficult. Over four weeks, there were intense debates involving the president, Cabinet members, lower-level officials and even former administration officials.

Obama was concerned that releasing the information could endanger ongoing operations, American personnel or U.S. relationships with foreign intelligence services. CIA officials, in particular, needed reassuring, the officials said.

But in the end, the view of the Justice Department prevailed, that the FOIA law required the release and that the government would likely be forced to do so by the court if it didn't do so itself, the officials said.

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 itself no longer exists.

He said "exceptional circumstances surround these memos and require their release" and does not change his determination to keep other intelligence operations secret and information about them classified.

But, said Obama, "Withholding these memos would only serve to deny facts that have been in the public domain for some time. This could contribute to an inaccurate accounting of the past, and fuel erroneous and inflammatory assumptions about actions taken by the United States."

Those assurances are not likely to innoculate Obama against criticism from conservatives. Last month, Vice President Dick Cheney said, for instance, that Obama's decisions to revoke Bush-era terrorist detainee policies will "raise the risk to the American people of another attack."