The United States Justice Department has ended its investigation into CIA interrogations of terrorist detainees without bringing criminal charges.

The decision in the probes 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 W. Bush.

Durham has looked into the treatment of 101 detainees in US custody since the September 11, 2001, terrorist attacks.

His probe into another episode involving the CIA began in January 2008 and was expanded in August 2009 to include a preliminary review of the CIA’s interrogation of specific detainees overseas.

In June 2011 Durham was given approval to move into a full criminal investigation of the 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 September 11 suspect’s children and suggested another would be forced to watch his mother being 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.

Attorney-General Eric 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”.

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

“Continuing impunity threatens to undermine the universally recognised prohibition on torture and other abusive treatment,” Jaffer said.

Durham’s review examined whether CIA interrogators used any unauthorised interrogation techniques, and if so, whether the techniques could constitute violations of the torture statute or any other laws.

The approach taken in the probe 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.