The memos may justify CIA techniques used at sites like Guantanamo Bay
The US has published four secret memos detailing legal justification for the Bush-era CIA interrogation programme.
Critics of the programme say the methods used amounted to torture.
President Barack Obama has also issued a statement guaranteeing that no CIA employees will be prosecuted for their role in the interrogation programme.
Some in the CIA wanted parts of the memos to be blacked out, fearing full disclosure could trigger lawsuits against agents, reports suggest.
The release of the memos stems from a request by civil rights group the American Civil Liberties Union (ACLU).
Three of the documents were written in May 2005 by the then acting head of the Justice Department's Office of Legal Counsel (OLC), Stephen Bradbury.
They gave legal support for the combined use of various coercive techniques, and concluded that the CIA's methods were not "cruel, inhuman or degrading" under international law.
The fourth document, dating from 1 August 2002, was written by OLC lawyer John Yoo and signed by his colleague Jay Bybee.
It contained 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.
Announcing the publication of the memos, Mr Obama said: "I believe that exceptional circumstances surround these memos and require their release.
"Withholding these memos would only serve to deny facts that have been in the public domain for some time," he explained.
But he also gave an assurance that "those who carried out their duties relying in good faith upon legal advice from the Department of Justice... will not be subject to prosecution."
The memos show for the first time the extent and the substance of the Bush administration's assurances to its intelligence operatives that what they were doing was not torture, and not against the law, says the BBC's Adam Brookes in Washington.
Mr Obama has outlawed the CIA's use of harsh interrogation techniques
Critics of the Bush administration's interrogation programme say the memos provide evidence that many of the methods amount to torture under US and international law.
There was a rift within the Obama administration about whether the documents should be made available to the public in full or should be partially redacted.
Attorney General Eric Holder and White House Counsel Greg Craig were vocal supporters of full publication of the memos, according to reports.
But CIA chief Leon Panetta and deputy director John Brennan called for portions of the memos to be blacked out, or redacted, the New York Times reported.
The intelligence officials were concerned that full disclosure would set a precedent for future exposure of the agency's sources and methods, and would threaten America's relationship with allied intelligence services.
But civil liberties campaigners said anything short of full publication would undermine President Obama's attempts to paint himself as more transparent than his predecessor.
During his first week in office, President Obama issued an executive order officially outlawing the use of harsh interrogation techniques by the CIA, and forcing the agency to adhere to standards laid out in the US Army Field Manual.