The US justice department told the CIA in 2002 that its agents would not be prosecuted for carrying out harsh interrogations if they believed they would not cause "prolonged mental harm", according to a memo written by a senior official.
The memo, released on Thursday by a civil rights group, approved the CIA's harsh interrogation techniques method by method.
The August 1, 2002 legal opinion, signed by Jay Bybee, the then assistant attorney-general, was obtained by the American Civil Liberties Union along with other documents.
The heavily censored document specifically approved proposed interrogation techniques that were devised for use against al-Qaeda suspects who were resistant to other questioning methods.
However, it also warned that if circumstances changed, interrogators could be prosecuted under anti-torture laws.
The standards used to judge how physically rough an interrogation was were censored.
"The healthier the individual, the less likely that the use of any one procedure or set of procedures will result in prolonged mental harm."
assistant attorney-general who wrote memo
But interrogations that stressed a detainee psychologically or emotionally were not allowed to cause "prolonged mental harm".
The memo suggested psychiatrists or psychologists should be consulted prior to interrogations to assess the likely mental health effect on the prisoner.
"The healthier the individual, the less likely that the use of any one procedure or set of procedures will result in prolonged mental harm," the memo stated.
Bybee wrote the memo the same day he wrote one for Alberto Gonzales, the then-White House counsel, that defined torture as only those "extreme acts" that cause pain similar in intensity to that caused by death or organ failure.
The Bybee legal opinion defining torture was withdrawn more than two years later.
For several years, the Bush administration relied on the findings issued in 2002 to maintain its interrogations did not amount to torture and had not violated any US or international treaties on treatment of detainees.
The Bush administration maintains waterboarding, which simulates the sensation of drowning, was legal when it was used by CIA interrogators in 2002 and 2003 against al-Qaeda suspects Khalid Sheikh Mohammed, Abu Zubaydah and Abd al-Rahim al-Nashiri.
Michael Mukasey, the US attorney-general, has refused to rule on the legality of waterboarding, saying he would only do so if the CIA sought to use it again.
George Bush, the US president, has vetoed an attempt to ban its use.
The practice has been widely condemned by international human rights groups.