The head of the US Senate intelligence committee believes a secret report into the CIA's actions in the aftermath of the September 11 terrorist attacks will be made public within weeks.

The report is believed to chronicle in unprecedented detail the extreme methods used by the CIA against terrorism suspects, tactics that critics have labelled torture.

Senator Diane Feinstein handed her report to the White House in April in the hope that it would be released within 30 days, but she is now expecting it will be released next month.

"I have been assured by the director of national intelligence that they should finish the declassification sometime around July 4," she told the ABC's Lateline program.

"Now, whether that's a moving target or not I don't know, but I think it'll be this summer (US time) for sure."

The imminent release of the report is being welcomed by family members of some victims of the terrorist attacks.

"I think the United States lost credibility internationally from some of the choices we made after 9/11. Hopefully Senator Feinstein's report gets us back on track," the co-founder of 9/11 Families for Peaceful Tomorrow, Colleen Kelly, said.

"Not only was there lack of transparency about what was happening, there was no debate, there was no discussion about what was being done in the name of Americans," said Ms Kelly, who lost her brother Bill in the attacks.

CIA made mistakes that 'haunt us to this day'

For five years Senator Feinstein's committee staff have been pouring over CIA internal emails, cables and other documents to research details of what the spy agency did to detainees.

The CIA had even set up a special computer room in a secure office near its Virginia headquarters to allow the oversight committee staff access to its material.

What went on in that room is now subject to a department of justice investigation about claims the CIA spied on the senator's staff and counterclaims from the agency, accusing her staff of stealing secret documents.

But that investigation has been overshadowed by what is expected to be revealed soon, with Senator Feinstein offering a blunt assessment of what her staff has uncovered.

"The report exposes brutality that stands in stark contrast to our values as a nation," she said in April.

"It chronicles a stain on our history that must never be allowed to happen again. This is not what Americans do.

"It is now abundantly clear that, in an effort to prevent further terrorist attacks after 9/11 and bring those responsible to justice, the CIA made serious mistakes that haunt us to this day."

It has since been reported that CIA agents went beyond waterboarding detainees and began to dunk suspects in ice baths, all the while concealing their activities from Congress and the White House.

Techniques 'sadistic and terrifying'

Those who know their actions are recorded in this report are dismissive of its findings.

"I think it's essentially a political document," John Rizzo, former head lawyer for the CIA, told Lateline.

Mr Rizzo is now out of the agency where he worked for 34 years. He rose to become its top lawyer when he received approval for what the world now knows as "enhanced interrogation techniques".

Mr Rizzo says he expects the intelligence committee report will conclude that the CIA broke the law.

"Well, yeah, I haven't seen it, but they seem to be heading in that direction," he said.

"In those early years, I was there, no-one on the hill expressed any objection to the [interrogation] program [when it] was briefed to the senior leadership of the Congress, much less called it a stain on the society."

Mr Rizzo now regrets not informing more politicians and members of the Bush administration about the program.

"If we had done that we would've gotten, for better or worse, the Congress on record as supporting this program. And [the fact I didn't] do that, I think was a huge tactical mistake for which I hold myself partly responsible."

In his book Company Man, Mr Rizzo describes some of the techniques as "sadistic and terrifying".

"Well, I was trying to be honest. Waterboarding to me, as it was first described to me, certainly sounded terrifying and on the surface it sounded sadistic. You're inflicting considerable hardship on a detainee," he said.

"What I didn't know when I was hit with those descriptions was whether these things crossed the line into torture. But I'm not going to soft soap it. I didn't in my book and I didn't at the time because these were nothing to be dismissed easily."

Civil rights group claims CIA has 'become rogue'

The American Civil Liberties Union (ACLU) has been trying for years to expose the CIA's treatment of detainees at secret black site prisons across the world such as Diego Garcia, Thailand and Poland.

"I think what we're talking about here is an intelligence agency that has become rogue," said Hina Shamsi, director of the ACLU's national security project.

"The United States has never had a reckoning – a top-to-bottom accounting of who authorised what when and with what consequences. That remains very much necessary," she said.

"We know that the CIA went far beyond what was authorised and we've never grappled with what senior level officials do.

"Among the lies that it apparently told those charged with conducting oversight was the CIA claimed that it had obtained information through torture which had actually been obtained using lawful interrogation before torture ever started."

Ms Shamsi also rejects the argument that torture can be used to get intelligence to stop imminent attacks, which is often cited as a defence of torture.

"You might get some accurate information through the use of torture, no-one denies it, but the reality also is that once you start using torture you do not know what is true and untrue," she said.

"Torture is not just [ineffective], immoral – it is counter-productive."

CIA officials unlikely to stand trial

Despite the death of her brother in the terrorist attacks, Ms Kelly believes there was no justification for torturing detainees.

"I understand that there were people in the CIA and other agencies who felt it was their duty and felt it was the right thing to do to torture people," she said.

"[But] it was torture. There's no question of what it was. It was torture."

But Mr Rizzo sees it differently.

"I mean there were a large number of 9/11 families who heartily endorsed the kinds of measures the CIA carried out to prevent future families from going through the same heartache and agony that they went through," Mr Rizzo said.

He believes that inflicting suffering on a few individuals in order to prevent terrorist attacks is justified.

"Yeah, the suffering of a few hardcore terrorists intent on attacking and killing Americans again for the benefit of the many potential victims of that attack – sure," he said.

Despite the release of the report, it is unlikely that any CIA officials will ever stand trial for the alleged torture.

President Barack Obama has said no-one from the CIA will be prosecuted for their actions, while The New York Times reported in 2012 that the department of justice would not prosecute agents who were part of interrogations that led to the deaths of two detainees.

Previously, the US attorney-general had also declared that no-one would face charges related to the use of enhanced interrogation techniques.