The U.S. Justice Department said on Thursday it has closed a criminal investigation into the death of two detainees in the custody of the Central Intelligence Agency (CIA) at overseas locations without charges.

Attorney General Eric Holder said in a statement that U.S. Attorney John Durham completed the investigation and the department declined prosecution "because the admissible evidence would not be sufficient to obtain and sustain a conviction beyond a reasonable doubt."

Two weeks ahead of the 11th anniversary of Sept. 11, 2001 terrorist attacks, Thursday's announcement marked the closure of the last active criminal investigation into CIA's treatment of prisoners since the attacks.

Over the past nearly five years, Durham has been leading the criminal investigation into the treatment of 101 detainees in U.S. custody since the Sept. 11, 2001 attacks. The federal prosecutor determined that many of the detainees were never in CIA custody, said Holder.

Durham's initial task was to look into the CIA's destruction of interrogation videotapes, the inquiry of which was also closed with no criminal charges. In August, 2009, based on information of alleged CIA mistreatment of detainees, Holder expanded Durham's mandate to a preliminary review of the CIA's interrogation of specific detainees at overseas locations.

In June, 2011, Holder approved Durham's request of a full criminal investigation regarding the death of two individuals while in U.S. custody at overseas locations.

One of the two cases involved the death of Gul Rahman, an Afghan who died in November, 2002, in a CIA facility in Afghanistan known as the Salt Pit. The second involved Manadel al-Jamadi, an Iraqi who died in 2003 at the notorious Abu Ghraib prison in Iraq.

The CIA has been under strong criticism for its mistreatment and abuse of prisoners after Sept. 11. Documents showed that the CIA had employed "enhanced techniques" to varying degrees, including stress positions, extreme temperature changes, sleep deprivation and waterboarding in the interrogation of some detainees.