Detainees at Guantanamo Bay have regularly complained of being tortured
Human and civil rights groups in the US have expressed dismay at news that CIA agents will not face prosecution over interrogation tactics in the Bush era.
Campaigners welcomed the White House's decision to publish details of harsh interrogation techniques now banned by President Barack Obama.
But rights groups said the decision not to prosecute agents was a failure to uphold the law of the land.
Others defended the techniques, saying the interrogations boosted US security.
The former head of the CIA, Michael Hayden, who ran the agency under President George W Bush, said the White House move would undermine intelligence work and dissuade foreign agencies from sharing information with the CIA.
"If you want an intelligence service to work for you, they always work on the edge. That's just where they work," he told the Associated Press.
Mr Obama banned the use of interrogation techniques such as sleep deprivation and simulated drowning - known as waterboarding - during his first week in office.
Aimed at simulating sensation of drowning. Used on alleged 9/11 planner Khalid Sheikh Mohammed
Harmless insect to be placed with suspect in 'confinement box', suspect to be told the insect would sting. Approved for Abu Zubaydah, but not used
Detainee slammed repeatedly into false wall to create sound and shock
Detainee shackled standing up. Used often, once for 180 hours
CIA agents are now required to use only those methods outlined in the US Army Field Manual.
The justice department has now released four memos detailing techniques the CIA was able to use under the Bush administration.
In publishing the memos, the Obama administration is clearly hoping to draw a line under the whole episode, says BBC defence correspondent Rob Watson.
The administration did not say it would protect CIA agents who acted outside the boundaries laid out in the memos, or those non-CIA staff involved in approving the interrogation limits.
That leaves open the possibility that those who crafted the legal opinions authorising the techniques, one of whom is now a federal judge, could yet face legal action.
Tom Porteous of Human Rights Watch says commanders could still be prosecuted
But politically and legally further moves will not be easy, our correspondent adds: the Department of Justice has already in effect ruled out any criminal prosecutions and is even offering legal assistance to any CIA official subject to any international inquiries or congressional investigation.
Politically the Obama administration has been sending strong signals that a functioning CIA is vital to national security and that nothing should be done to weaken confidence or morale at the agency, our correspondent says.
Announcing the release of the four memos, Attorney General Eric Holder said the US was being "consistent with our commitment to the rule of law".
"The president has halted the use of the interrogation techniques described in these opinions, and this administration has made clear from day one that it will not condone torture," he said.
However, civil and human rights campaigners remain uneasy and there are calls for an independent inquiry into the issue, one which could offer immunity from prosecution in return for testimony.
The release of the memos stems from a request by civil rights group the American Civil Liberties Union (ACLU).
Amnesty International said the Department of Justice appeared to be offering a "get-out-of-jail-free card" to individuals who were involved in acts of torture.
"Bottom line here is you've had crimes committed," Amnesty International analyst Tom Parker told the BBC.
"These are criminal acts. Torture is illegal under American law, it's illegal under international law. America has an international obligation to prosecute the individuals who carry out these kind of acts."
One of the four secret memos released on Thursday contains legal authorisation for a list of specific harsh interrogation techniques, including pushing detainees against a wall, facial slaps, cramped confinement, stress positions and sleep deprivation.
The memo also authorises the use of "waterboarding", or simulated drowning, and the placing of a detainee into a confined space with an insect - one technique that was not eventually used.
However, most methods were regularly used on "high value" detainees suspected of belonging to the upper echelons of al-Qaeda's command structure.
Jed Babbin, a former Pentagon official during President Bush's first term, told the BBC that the decision to use harsh methods was justifiable.
"I'm not at all convinced that what we got from them was not valuable intelligence in some circumstances," he said, adding that "retribution" against Bush-era officials would not be productive.
"The job of the American justice system is not to please human rights groups, it is to administer the American law as it exists."
REACTIONS TO OBAMA DECISION
The memos' matter-of-fact clinical descriptions belie the harsh tactics to which they gave a green light. But... it is not enough to say that when we have a president who does not believe in cruel and inhuman treatment and torture, the United States will not engage in such practices. We must formally acknowledge that what was done was wrong, indeed criminal.
By repudiating the memos, the Obama administration has again seized the high ground and restored some of the honor lost over the past few years. Yet the decision to forgo prosecutions should not prevent -- and perhaps should even encourage - further investigation about the circumstances that gave rise to torture.
On the surface, the statement today looks like a big ol' grant of immunity - or a concession - or a deliberate attempt to boost morale at the CIA... There are plenty of CIA officers who followed the rules and shouldn't be prosecuted. They're the ones who are a little relieved today... although they might have to explain some things to their priests and their families.
Another major issue is lingering, however. Did the torture "work"?... Without a rigorous investigation into the alleged efficacy of U.S. torture, we'll never know. But while Obama has turned the page, many others haven't - including the people, and their allies, who think waterboarding was a good idea. Without a commission... we could start torturing all over again.
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