As Holder's decision to launch an investigation followed a recommendation by the US justice department to consider re-opening several cases of prisoner abuse alleged to have been carried out by CIA employees or contractors in Iraq and Afghanistan.

"I fully realise that my decision to commence this preliminary review will be controversial," Holder said in a statement.

"In this case, given all of the information currently available, it is clear to me that this review is the only responsible course of action for me to take."

'Limited scope'

But Jayne Huckerby, research director at the Centre for Human Rights and Global Justice at New York University, said that Holder had not made the scope of the investigation wide enough.

In depth

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"The attorney-general has indicated that it will be a preliminary review into whether federal law was violated in respect of specific interrogations of particular detainees overseas," she told Al Jazeera.

"On its very terms, [Holder] has limited the scope of the inquiry. Added to the concerns of who will be testifying, what documents will be accessed, it is very concerning that the inquiry will be limited.

"That is a particularly stark concern, given the other thing that happened today - the release of the 2004 CIA inspector-general's report into secret detention facilities and the interrogation techniques that were used there."

The CIA report, written by the agency's inspector-general five years ago but made public only on Monday, said interrogators used "unauthorised, improvised, inhumane and undocumented" techniques.

The so-called "enhanced" interrogation techniques that critics have called torture, went beyond sleep deprivation, withholding food, and waterboarding, where a suspect is made to feel like he is drowning.

Interrogators threatened to kill the children of Khalid Sheikh Mohammed, the alleged mastermind of the September 11, 2001 attacks on the US, and implied that another suspect's mother would be sexually assaulted.

Some officers also fired a gun in a room adjacent to where a prisoner was being held to make him believe another suspect had been executed, the report said.

Under US law it is illegal to threaten a detainee with imminent death.

Alexander Abdo, from the American Civil Liberties Union's national security project which went to court to get the CIA report made public, told Al Jazeera that his organisation hopes "that agents hesitate in the future before they resort to techniques that amount to torture".

"We think that is a form of hesitation that is beneficial. It is good for the reputation of the United States, it is good for our PR in parts of the world that might want to do us harm and it is also good in terms of the effectiveness of the interrogation technique.

"The [US] president made clear in May that he believes that the most effective techniques are not the coercive techniques of the CIA but the rapport-building techniques used by the FBI and other techniques used in the army field manual," he said.

New interrogation unit

On Monday, Barack Obama, the US president, approved the formation of a White House-supervised unit to interrogate terrorism suspects.

Bill Burton, the deputy White House spokesman, said the president had set up the High Value Interrogation Group, which will be housed at the FBI and answer to the National Security Council.

It will adhere to interrogation guidelines based on the US army field manual, which forbids techniques such as waterboarding, Burton said, adding that the CIA will no longer handle the questioning of people suspected of planning or carrying out attacks.

"The president's view is that intelligence gathering is best left to the intelligence community," he said.

The developments on Monday could expose CIA employees and contractors to prosecution for their treatment of suspects.

Obama has said that those who interrogated suspects based on legal guidelines written by the administration of his predecessor George Bush should not face legal action, but Burton acknowledged that Holder has the final say.

"The president has said repeatedly, he thinks that we should be looking forward, not backward," Burton said.

"But, ultimately, the decisions on who is investigated and who is prosecuted are up to the attorney-general."