American Civil Liberties Union deputy legal director Jameel Jaffer said on Thursday that the the US Justice Department's decision not to bring charges over the deaths of terror detainees was "nothing short of a scandal."
His comments followed a probe by federal prosecutor John Durham into the treatment of two detainees who died in CIA custody.
Gul Rahman died in November 2002 at a CIA prison in Afghanistan known as the Salt Pit after being shackled to a wall.
Manadel al-Jamadi died at Abu Ghraib prison in Iraq in 2003 and a military post-mortem declared his death a homicide.
CIA officers had taken him to a shower stall, put a sandbag over his head, cuffed his hands behind his back and chained his arms to a barred window.
Within an hour, he was dead.
The officer in charge, who was reprimanded over the incident, now works for a private defence contractor.
The investigation followed widespread claims of abuses by CIA interrogators. One CIA inspector's report revealed that interrogators had threatened to kill one suspect's children and suggested that another would be forced to watch his mother sexually assaulted.
The report said some that some interrogators even went far beyond regulations that gave them wide latitude to use tactics such as holding prisoners in stress positions for extended periods, extreme temperature changes, sleep deprivation and waterboarding.
Attorney General Eric Holder said: "Based on the fully developed factual record concerning the two deaths, the department has declined prosecution because the admissible evidence would not be sufficient to obtain and sustain a conviction beyond a reasonable doubt."
Predictably, CIA sources hailed the decision as a victory.
Former CIA director Michael Hayden said he was "heartened that the investigation is complete and heartened by the results.
"I had great confidence in Mr Durham. I just regret that many CIA officers had to go through yet another review of these activities."
But Mr Jaffer warned that "continuing impunity threatens to undermine the universally recognised prohibition on torture and other abusive treatment."
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