The US justice department has announced it has ended its investigation into CIA interrogations of terrorist detainees without bringing criminal charges.

The decision in the inquiries of the deaths of two terrorist suspects marks the end of a wide-ranging criminal investigation by federal prosecutor John Durham into interrogation practices during the presidency of George Bush.

Durham has looked into the treatment of 101 detainees in US custody since the 9/11 terrorist attacks. Durham's inquiry into another episode involving the CIA began in January 2008 when the justice department chose him to conduct a criminal investigation into the agency's destruction of videotapes it had made of its interrogations of terrorist suspects.

In August 2009, attorney general Eric Holder expanded Durham's mandate to include a preliminary review of the CIA's interrogation of specific detainees overseas. In June 2011, Holder approved Durham's request to move into a full criminal investigation of the two deaths.

The 2009 expansion followed the public release of an internal CIA inspector general's report that revealed agency interrogators once threatened to kill a 9/11 suspect's children, and suggested another would be forced to watch his mother be sexually assaulted. The report said some CIA interrogators went beyond Bush administration restrictions that gave them wide latitude to use severe tactics such as waterboarding, a simulated drowning technique.

About the just-completed investigation of the two detainees' deaths, Holder said that "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."

In a message to employees Thursday, CIA director David Petraeus said that "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" and "I would like to thank everyone who played a role" in doing so.

Former CIA director Michael Hayden said he was "heartened that the investigation is complete, and I'm 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."

Jameel Jaffer, deputy legal director of the American Civil Liberties Union, called the outcome of the investigation "nothing short of a scandal."

"Continuing impunity threatens to undermine the universally recognized prohibition on torture and other abusive treatment," Jaffer said.

Durham's review examined whether CIA interrogators used any unauthorized interrogation techniques, and if so, whether the techniques could constitute violations of the torture statute or any other laws. The approach taken in the inquiry was not to prosecute anyone who acted in good faith and within the scope of the legal guidance given by the justice department's Office of Legal Counsel regarding the interrogation of detainees.

Thursday's announcement came in the deaths of Gul Rahman and Manadel al-Jamadi. Rahman died in the early hours of 20 November 2002, after being shackled to a cold concrete wall in a secret CIA prison in northern Kabul, Afghanistan, known as the Salt Pit. He was suspected of links to the terrorist group al-Qaida. Rahman is the only detainee known to have died in a CIA-run prison.

Before Durham looked into Rahman's death, two other federal prosecutors conducted separate reviews and could not prove the CIA officer running the Salt Pit had intended to harm the detainee, a point made in a government document that has been released publicly.

Al-Jamadi died in 2003 at the Abu Ghraib prison in Iraq. A military autopsy declared al-Jamadi's death a homicide.

At Abu Ghraib prison, instead of turning al-Jamadi over to the Army, CIA officers took him to a shower stall. They put a sandbag over his head, cuffed his hands behind his back and chained his arms to a barred window. When he leaned forward, his arms stretched painfully behind and above his back.

Within an hour, he was dead.

At least three CIA employees came under scrutiny, including a paramilitary officer who ran what was known as the detainee exploitation cell at Abu Ghraib.

The officer was on the raid when a group of Navy Seals captured al-Jamadi. He processed al-Jamadi into the prison but he was not in the shower room when al-Jamadi died.

The officer failed to have a doctor supervise al-Jamadi before he was processed into the prison, violating agency procedures. The officer, who was reprimanded over the incident, now works for a defense contractor.