They approved techniques including waterboarding, week-long sleep deprivation, nudity and putting insects in with a tightly confined prisoner.
The first memo, from 2002, approves waterboarding - which makes the suspect feel like they are drowning - and other harsh techniques on suspected high-level al-Qaeda figure Abu Zubaydah.
It was written by former Assistant Attorney General Jay Bybee for the CIAs top lawyer, John Rizzo. The CIA has confirmed that two other high-level al-Qaeda suspects were waterboarded.
The memo says: "We find that the use of the waterboard constitutes a threat of imminent death" - one of the criteria for torture.
"It creates in the subject the uncontrollable physiological sensation that the subject is drowning."
But it also said that in the absence of prolonged mental harm, no severe mental pain or suffering would have been inflicted, and the use of these procedures would not constitute torture within the meaning of the statute.
The memo described waterboarding as the final point in a chain of harsh techniques, which also including slapping and shoving a suspect into a wall, aimed at persuading Zubaydah to reveal information interrogators were certain he had.
One memo said 28 terrorism suspects received harsh interrogations out of 94 held in the CIAs detention program.
US President Barack Obama has banned the use of extreme interrogation but said agents who used the techniques on suspects would not face punishment.