The US justice department has ended its investigation into the CIA's interrogation programme for terror detainees, without bringing charges.
The probe, which studied two deaths in Afghanistan and Iraq, ends a series of reviews into the treatment of 101 detainees in US custody since 9/11.
Attorney General Eric Holder said there was not enough evidence to "sustain a conviction beyond a reasonable doubt".
A prominent civil rights group said the result was a "scandal".
But CIA officials welcomed the decision. They have always maintained that the interrogation programme was conducted according to guidelines issued by lawyers in George W Bush's administration.
In June 2011 the justice department began a full criminal investigation into the deaths of two men while in custody.
The department has never named the men, but they are widely reported to have been Gul Rahman and Manadel al-Jamadi.
Gul Rahman died in November 2002 at a CIA-run prison in Afghanistan known as the Salt Pit after being shackled to a concrete wall.
Manadel al-Jamadi died at Abu Ghraib prison in Iraq in 2003, and a military post-mortem examination declared his death a homicide. Some reports said he had been beaten by US Navy Seals, others said he had been chained to a high barred window with a sandbag over his head.
In a statement, Mr 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."
But he added that the department of justice investigation "was not intended to, and does not resolve, broader questions regarding the propriety of the examined conduct".
The investigation, which was launched in 2008 by federal prosecutor John Durham, had a remit of establishing whether the treatment of detainees broke guidelines issued by lawyers of the administration of former President George W Bush.
Under Mr Bush's presidency that guidance included a variety of techniques, including "waterboarding" or simulated drowning, now officially regarded as torture by the Obama administration.
The inquiry was originally set up to investigate why videotapes of CIA interrogations had been destroyed.
But the attorney general later expanded the probe to investigate the detention and interrogation of 101 detainees alleged to be in US custody in the years after the attacks of 11 September 2001.
The decision followed the public release of an internal CIA inspector general's report in 2009 that revealed interrogators had threatened to kill and possibly sexually assault members of detainees' families.
The report detailed basic abuses such as such as stepping on shackles, washing someone with a wire brush and blowing smoke constantly in a detainee's face and suggested some interrogators may have broken the Bush administration's guidelines.
In 2011, Mr Durham then recommended the justice department pursue a full criminal investigation into the two deaths in custody.
CIA Director David Petraeus thanked his staff for co-operating with the investigation.
"As intelligence officers, our inclination, of course, is to look ahead to the challenges of the future rather than backwards at those of the past. Nonetheless, it was very important that we supported fully the Justice Department in its efforts," he said.
But Jameel Jaffer, deputy legal director of the American Civil Liberties Union, said the investigation's conclusions were a "nothing short of a scandal".