Pressure is building on the CIA to release one of the most potentially explosive documents of the post-9/11 era: a secret Senate report revealing in gruesome detail the extent of the agency’s use of torture.

In one corner is the US’s premier intelligence agency, which has labored for years to put its history with torture behind it. In the other is a Senate committee whose dogged, even angry pursuit of a grim episode has placed it against an agency to which it ordinarily gives the benefit of the doubt – all while it is trying to convince the public it can effectively police the National Security Agency.

In the balance is something that, for civil libertarians, has been elusive: accountability for torture.

“There are certain smart people at the CIA who have to realize that if that kind of information becomes public, it’s going to renew the calls for a criminal investigation,” said Chris Anders, a Washington-based attorney with the American Civil Liberties Union (ACLU).

Almost exactly a year ago, the Senate intelligence committee approved, by a 9-6 bipartisan vote, a 6,300-page inquiry into brutal interrogations conducted by the CIA on suspected al-Qaida detainees. The years-long study documented not only the practices behind euphemisms like “waterboarding” and “stress positions”, but what senators have described as useless information yielded by them and deceptions by the CIA to Congress about their importance.

The public has never seen the report. The CIA has feuded with a committee that is deeply inclined toward giving it and its partner intelligence services the benefit of the doubt, as accusations of deception have swirled between Langley and Capitol Hill. The unresolved tension has already cast a shadow over the brief CIA directorship of John Brennan, a confidante of President Barack Obama who was a senior agency official in the period of the events that comprise much of the report’s focus.

On Tuesday afternoon, the battle emerged into full view during a nomination hearing for the CIA’s next top lawyer. Angry senators once again excoriated Langley for stalling on declassifications, and lashed out over CIA statements to the press that the Senate report is factually flawed.

“I'm more confident than ever in the factual accuracy,” said Senator Mark Udall, who accused the CIA of misleadingly telling the Daily Beast that the CIA objected to the report when, he revealed, an internal agency study had found no such factual objections.

Hours later, however, the New York Times ran a piece containing a reiteration of the same CIA insistence that the Senate inquiry contained “significant errors” about what the agency prefers to call “enhanced interrogation”.

Asked about Udall’s claim, CIA spokesman Dean Boyd said: “We are aware of the committee’s request and will respond appropriately.”

What seems on the surface like an obscure dispute inside the Beltway has attracted combatants outside of it. This week, a coalition of religious groups organized an open letter to the committee, signed by Christian, Muslim and Jewish clergy across the country, describing the release of the report as a critical accountability measure.

“We hope that op-eds written by local religious leaders in publications in all 50 states will demonstrate to the Senate intelligence committee the depth of the opposition to torture within the religious community and our community’s support for sharing the truth about torture by releasing the report to the public,” said Reverend Richard Killmer, the executive director of the National Religious Campaign Against Torture, in a statement.

For Brennan, the torture issue has overtaken his tenure. An agency veteran and an Arabic speaker, he lost the CIA directorship shortly after Obama’s election, due to statements he made in old press interviews that condemned waterboarding – forcing water over a detainee’s nostrils and mouth, preventing them from breathing – but suggested that “enhanced interrogation techniques” saved lives, a point under fierce dispute.

Brennan became the White House counterterrorism director instead, an arguably more powerful position that gave him practically unmediated access to Obama. While Brennan was scrutinized more for approving drone strikes, the torture issue re-emerged once Obama definitively nominated him to run the CIA.

His February confirmation hearing featured Brennan having to forswear internal knowledge of torture; insisting, contrary to contemporaries’ recollections, that he attempted to stop it; and pledging to work with the committee on a torture report he said he had not fully read.

“I very much look forward to hearing from the CIA on that and then coming back to this committee and giving you my full and honest views,” Brennan told the committee chair, Dianne Feinstein. He was confirmed soon after.

By June, however, Brennan delivered a classified “rebuttal” to the Senate panel, contesting its conclusions and its command of the facts. Former agency officials insisted that the Senate, not the agency, was playing politics with the efficacy of torture. “I don’t know how they could fail to say that actually it was effective,” one anonymous ex-CIA official told the Washington Post.

At particular issue are undisclosed legal memorandums, written in secret at the Justice Department, that gave torture the imprimatur of legality.

Feinstein and other senators have complained for over a year that neither the CIA nor the Justice Department has shared such memorandums with the committee. On Tuesday, the senators reiterated a contention that the CIA did not just lie to the committee about the value of torture, but it also lied to the Bush-era Justice Department office of legal counsel about how it actually implemented the brutal techniques the department blessed.

Yet Caroline Krass, a top lawyer in the office of legal counsel, whom Obama nominated to become the CIA’s chief attorney, told the panel on Tuesday that the Senate panel was not entitled to the memorandums, which she described as “pre-decisional” and therefore beyond Senate prerogative.

Those legal memos could thrust the CIA back into a minefield it thought Obama had saved it from facing. Even before his inauguration, Obama signaled he had no interest in charging anyone involved in torturing detainees with a crime. “We need to look forward as opposed to looking backwards,” he said, in a January 2009 interview.

An independent Justice Department investigation deliberately excluded from its purview any senior official or lawyer who authorized the torture, and examined instead about 100 instances in which low-level CIA officials and contractors carried out the brutal interrogations. In 2012, the inquiry formally ended – without prosecuting anyone.

It was an enormous relief for a CIA that has struggled to put its torture legacy behind it, despite whispering to the filmmakers of the controversial Hollywood hit Zero Dark Thirty that torture was a critical counterterrorism practice.

But if the Senate report indicates that the CIA misled the Justice Department, said Anders of the ACLU, the agency might find itself under renewed calls to reopen an inquiry aimed at prosecution.

Anders said: “The most significant way that the CIA has been protected against prosecution has been its claims that it was relying on legal advice from the Justice Department, but if the very basis of that legal advice was misrepresented facts provided by the CIA, then by relying on those legal opinions when you know it’s based on false or incomplete information, the CIA undermined the value of those opinions.”

There are also high stakes for the Senate intelligence committee.

The disclosures from Edward Snowden about the breadth of NSA surveillance has damaged public confidence in Congress’s ability to effectively oversee the intelligence agencies, on issues ranging from the bulk collection of Americans’ phone data to overseas “targeted killing” launched from CIA drones. Now the CIA is publicly stonewalling the committee, effectively defying the panel on what chairwoman Feinstein pointedly told Krass on Tuesday was its sole “purpose”.

Feinstein and other committee members are procedurally encumbered by the CIA. The committee is bound by rules that prevent it from making unilateral declassification decisions. Votes to compel public releases would involve the rest of the Senate, and the executive branch has broad leverage over declassification.

At the end of Tuesday’s hearing, Feinstein urged Krass to let her staff “sit down with you and explain to you exactly what it is and that you take some action” over the documents the committee wants. Krass, equivocally, said she wanted to “work closely with this committee to make sure that you get access to the information as appropriate”.

The episode occurs as Congress is considering restrictions on the NSA’s surveillance programs, an effort that has new legislative urgency after a federal judge this week said bulk collection of US phone data was likely unconstitutional and a White House review panel recommended curbing aspects of NSA surveillance.

An argument that unites NSA and the committee leadership is that the bulk surveillance has been under Congressional oversight, which they hope reassures the public that the broad surveillance is wise, legal and proper. The rare public viewing of the CIA’s resistance to such oversight, Anders said, has complicated that arguement.

“You realize just how little oversight is going on with Congress,” Anders said. “It’s like a pretty-please-can-you-give-us-something attitude.”