The CIA carried out "brutal" interrogations of al-Qaeda suspects in the years after the 9/11 attacks on the US, a US Senate report has said.
The summary of the report, compiled by Democrats on the Senate Intelligence Committee, said the CIA had misled Americans about what it was doing.
The information the CIA collected this way failed to secure information that foiled any threats, the report said.
In a statement, the CIA insisted the interrogations had helped save lives.
"The intelligence gained from the programme was critical to our understanding of al-Qaeda and continues to inform our counterterrorism efforts to this day," director John Brennan said.
However, the CIA also acknowledged mistakes in the programme, especially early on when it was unprepared for the scale of the operation to detain and interrogate prisoners.
The programme - known internally as Rendition, Detention and Interrogation - took place from 2002-07, during the presidency of George W Bush.
Suspects were interrogated using methods such as waterboarding, slapping, humiliation, exposure to cold and sleep deprivation. Such methods were referred to as "Enhanced Interrogation Techniques" (EIT).
Analysis: Frank Gardner, BBC security correspondent This report makes deeply uncomfortable reading but it shines a much-needed torch into some dark places.
The fact that Enhanced Interrogation Techniques, or torture by any other name, was stopped years ago or that some people at the top of the US administration may not have known its full extent, does not excuse the fact it took place at all.
After going through six million pages of documents, the authors concluded that in none of the cases they had looked at did these brutal methods stop a terrorist attack. Meaning that America's reputation, and by extension that of the wider West, has been sullied for no tangible gain.
This will lay the US open to charges of hypocrisy, making it far harder for the West to criticise brutal and dictatorial regimes. It may also encourage terrorists to justify their atrocities by pointing to this past abuse.
It can only be hoped this report's publication means these practices will be consigned to history's dustbin.