In February 2009, the Senate intelligence committee gathered in a soundproof room to learn the stomach-churning details of the brutal interrogations the CIA conducted with its first important al-Qaida prisoners.

Committee aides distributed a report based on a review of messages to CIA headquarters from two of the agency's secret overseas jails. Included was a 25-page chart with a minute-by-minute description of 17 days during which the first detainee, Abu Zubaydah, was kept awake, slammed into walls, shackled in stress positions, stuffed for hours into a small box and waterboarded to the point of unconsciousness.

The captive ranted, pleaded and whimpered, an accompanying text said, but he never provided the information about brewing terrorist plots that the CIA thought he had.

Senators were aghast. Some muttered that such horrific acts by Americans should never see the light of day, recalled aides who spoke on condition of anonymity because they could not publicly discuss a classified session. Other senators voiced outrage over how this new account differed from the antiseptic CIA descriptions of "enhanced techniques."

A few weeks later, the committee voted, 14-1, to begin a full investigation into the CIA's post-Sept. 11 detention and interrogation practices.

The resulting report, a summary of which was released in December, was a rare instance of an oversight committee seeking to hold the CIA accountable in a public way. It also was the most detailed critique of the CIA in a generation.

And it raised the question of how well other covert CIA programs are managed, from targeted killing with drones to the secret effort to train and arm Syrian rebels.

Congressional intelligence committees long have been accused of being "captured" by the agencies they oversee. When they do expose and correct problems, it usually happens behind closed doors.

Even for those who dispute some of the central conclusions, the 518-page summary of the 6,770-page classified study shows how a rigorous examination of a secret agency can expose misconduct, incompetence and bureaucratic spin.

Based on a review of 6 million pages of CIA documents, the classified report covers 12 bound volumes. Senate investigators pored over records few outsiders ever see. Cables, chats, emails, internal memos and interview transcripts detailed not just the official story but the messy behind-the-scenes threads, including internal criticism and office gossip.

The early bipartisanship quickly faded. Republicans withdrew from the investigation and at times actively opposed it.

Written by Democrats working for California Sen. Dianne Feinstein, then the Senate intelligence committee's chairwoman, the report concluded that the brutal CIA interrogations failed in every case to produce unique, life-saving, otherwise unavailable intelligence, and that the CIA repeatedly distorted this fact.

In dissent, Republicans and CIA officials say the report cherry-picks evidence, obscures context and suffers from the absence of interviews, which were not possible because of a pending criminal investigation.

Still, the investigation forced the spy agency to publicly admit that it mismanaged the interrogation program, failed to punish misconduct and detained people it shouldn't have.

Yet the intelligence committees have never taken a similar look at what is now the premier counterterrorism effort, the CIA's drone-killing program, according to congressional officials who were not authorized to be quoted discussing the matter.

Intelligence committee staff members are allowed to watch videos of CIA drone missile strikes to monitor the agency's claims that civilian casualties are limited. But the aides do not typically get access to the operational cables, message traffic, interview transcripts and other raw material that forms the basis of a decision to kill a suspected terrorist.

Nor have they been able to examine cables, emails and raw reporting to investigate recent perceived intelligence lapses, such as whether the CIA failed to predict the swift fall of Arab governments, Russia's move into Ukraine or the rapid military advance of the Islamic State group.

And there have been no public oversight reports on the weak performance of the CIA's multibillion-dollar "nonofficial cover" program to set up case officers posing as businessmen, which has met with criticism.

The six-year Senate investigation "illustrates both the strengths and the weaknesses of the oversight process," said Steven Aftergood, an intelligence expert with the Federation of American Scientists. "It represented a massive allocation of effort, tens of millions of dollars, years of research, millions of documents, all to generate a narrative account that will dominate public understanding of CIA interrogation for decades to come."

But no one has been held accountable for the conduct described in the report, there is nothing in the law that would prevent the CIA from repeating it and there is no prospect of a similar investigation into other covert intelligence programs.

Since the underlying CIA records remain secret, there always may be disagreement about whether detainees who were brutalized subsequently provided information important to the hunt for Osama bin Laden, for example_even if it's clear that CIA got much of its intelligence in that case from other sources. An anti-torture advocate, Georgetown law professor David Cole, wrote recently that he wasn't convinced by the report's argument that harsh measures were ineffective in every instance.

The latest opinion polls find widespread support for the CIA's actions and for the notion that the government may occasionally need to torture terrorists.

But if the report did not immediately change public sentiments or settle the question of whether CIA torture worked, it clearly documented dozens of instances of the CIA exaggerating and misstating the fruits of these interrogations to justify its actions, including inaccurate testimony to Congress about the bin Laden case. CIA officials reject some of those criticisms but embrace others.

"In some instances, we simply failed to live up to the standards that we set for ourselves and that the American people expect of us," CIA Director John Brennan told reporters at CIA headquarters in December. It is difficult to imagine him uttering those words in the absence of the Senate report.

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A HISTORIC UNDERTAKING

The report's principal author, Dan Jones, spent years of long nights and weekends sifting through millions of classified electronic pages in a secure government facility in Tysons Corner, Virginia. Others heavily involved included Mike Davidson, the committee's former general counsel; David Grannis, the current staff director; aides Evan Gottesman and Chad Tanner; and Alissa Starzak, a former CIA lawyer now at the Pentagon.

Jones came to know the CIA's interrogation program better than any of the CIA officers who worked on it, colleagues say, because he could see the full picture over a period of years, including exactly what every detainee told his questioners, under what circumstances and how the information was used.

Buried in the millions of pages were details that even CIA directors didn't fully grasp, as evidenced by then-Director Michael Hayden's error-filled 2007 briefing on the program to the Senate intelligence committee that is painstakingly chronicled in the report's appendix. (Hayden maintains he was discussing the program as it had evolved in 2006, not the one that existed in 2002.)

The Senate's Church and the House's Pike committees of the 1970s applied similarly aggressive scrutiny to the CIA, said Loch Johnson, who worked on those investigations and wrote a book about the Senate probe.

But those investigators, who documented widespread intelligence agency misdeeds in a seminal effort that paved the way for the modern system of congressional oversight, couldn't dig as deeply. They were looking at hundreds of secret programs instead of one, and they were operating in an era before instant messages and digital search tools.

The Senate intelligence committee got the extraordinary access to internal CIA documents mainly because of the fateful decision by CIA operations chief Jose Rodriguez in 2005 to order the shredding of videotapes of some of the interrogations. When that destruction was revealed in 2007, Hayden, seeking to rebut charges of a cover-up, agreed to turn over the operational cables describing the interrogations.

From there, it was difficult for the CIA to resist further demands for more records. In the end, the committee got nearly everything it asked for, with the exception of 9,000 pages blocked by the White House on the grounds of executive privilege.

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OVERSIGHT CONTRADICTION

Michael Colaresi, a political scientist who has written about how Western democracies police their spy agencies, believes access to internal agency records should be a regular feature of congressional intelligence oversight.

The U.S. once led the way in keeping tabs on its spies, but now several Western nations conduct more thorough oversight of their intelligence agencies, he said. A Netherlands parliamentary commission, for example, employs outside experts who can demand documents, require witness testimony and issue public reports.

"Intelligence oversight is necessary to reassure the public that if there is bad policy being done behind the veil of classification, it's going to be revealed," he said.

But U.S. congressional intelligence oversight committees have neither the staff nor the time to regularly conduct in-depth investigations. More commonly, oversight consists of weekly secret hearings at which lawmakers or aides listen to top CIA officials talk.

Also, committee members tend to be big supporters of intelligence agencies.

When Edward Snowden revealed secret National Security Agency surveillance programs that disturbed many Americans, most intelligence committee members defended the programs. The lawmakers had been briefed on them in secret for years by NSA officials who believed they were crucial tools.

Feinstein embodies that contradiction.

She backed the NSA after Snowden, and she long has been known as a staunch defender of the CIA. The torture investigation, she said in an interview with The Associated Press, has "changed how I view management in the CIA. It's changed how I view the brotherhood of the CIA. I believe you do not lie to your oversight committee. And I think the way the program was managed was sloppy."

The lesson for traditional intelligence oversight, she said, was that "you can sit and listen to a report — you don't know whether it's all the truth, you don't know what gets left out. And part of (CIA) tradecraft is deception."

She said she believes the CIA continues to lie about the effectiveness of torture.

"Absolutely," she said. "The question is, what can you do about it?" The CIA declined to comment on her remarks.

Asked about CIA drones, however, she pivoted to a defense of the agency. Although her report includes examples of the CIA kidnapping the wrong person and brutalizing another detainee who was "not the man we thought he was," Feinstein expressed confidence the CIA has not made such mistakes with drones.

In killing with drones, "the CIA takes its time," she said. "They're very careful about the identification of the individual. And they're able to make that identification. Sometimes the intelligence gathering goes on for months."

Feinstein points out that congressional oversight of drones has been far more thorough than was the early oversight of interrogations. The George W. Bush administration gave a small number of cursory briefings about the CIA interrogation program beginning in 2002 to a few congressional leaders, while the Obama administration regularly has updated the intelligence committees about drone killings.

But drone opponents question whether Congress has been co-opted by a superficial semblance of oversight and is failing to push for answers to hard questions about drones, such as whether hatred engendered by the strikes outweighs their tactical benefits.

The findings on interrogations "do make you wonder what else we might not be conducting adequate oversight on," said Sen. Martin Heinrich, D-N.M.

That appears to be an academic question, because the Republicans who now control the Senate have expressed misgivings about public intelligence oversight.

Sen. Richard Burr of North Carolina, the new committee chairman, has asked executive branch agencies to give back copies of the torture report's classified version, in what is widely seen as a move to prevent its future release.

For the first time in many years, the Senate and House intelligence committees closed the normally public hearing in February featuring the CIA director discussing the threats facing the country.

Burr's spokeswoman, Rebecca Glover Watkins, pointed out that the committee recently held an open oversight hearing during which the chief of the National Counter Terrorism Center faced tough questions on a number of matters. The chairman plans to hold more open hearings than Democrats did in years past, she said.

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DISTORTIONS EXPOSED

CIA officials have argued that after a rough start, there was plenty of internal oversight of the interrogation program, including by the agency's inspector general, who wrote a critical report in 2004.

But the Senate investigation exposed misstatements by CIA officials as part of that inquiry that stood, uncorrected, for years. One example involves the case of Ali Saleh Kahlah al-Marri, the only person to be seized from the U.S. justice system, declared an enemy combatant and thrown in a military jail.

In 2003, when the inspector general was gathering information about the interrogation program, a CIA lawyer asked analysts to supply "a list of specific plots that have been thwarted" through the fruits of brutal interrogations, according to the Senate report.

In response, a senior CIA analyst sent an email rife with false information, the report says. Among the errors, she said that Sept. 11 mastermind Khalid Sheikh Mohammed, after being waterboarded and otherwise brutalized, identified a photograph as an al-Qaida operative named Saleh al-Marri, who was then arrested.

But, as the CIA finally acknowledged in its 2013 response to the Senate report, al-Marri had been in federal custody for 16 months before Mohammed was captured. He had been indicted in Illinois on credit card fraud and other charges, including lying to the FBI about his al-Qaida ties.

It was one of many such errors made by that same officer, who remains in a senior position at the agency. But the CIA argues that Mohammed's information was nonetheless crucial to the government's understanding of al-Marri, implying that the al-Marri case stands as a justification for brutal interrogations.

After Mohammed told interrogators al-Marri was sent to the U.S. to conduct a terrorist attack, the Bush administration had him placed in military detention under harsh conditions. He later pleaded guilty to terrorism charges in federal court. After serving the remainder of a 15-year sentence, he was released in January and deported to his native Qatar.

One of al-Marri's lawyers, Jonathan Hafetz, says it's ridiculous to suggest that torturing Mohammed stopped al-Marri. When the FBI arrested him they seized a laptop containing searches about cyanide, material about bin Laden, coded messages to Mohammed and communications with an al-Qaida figure known to have handled the Sept. 11 financing.

"The FBI was on to him," Hafetz said. "Had he been convicted on the original fraud and false statement charges, he faced a heavy sentence, potentially decades long."

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LAST-DITCH EFFORT TO STOP IT

Colaresi and other scholars argue that the full version of the report and some of the underlying documents should be released, so the public can sort through the competing claims by the CIA, Republicans and Democrats about what the records actually show.

But while Obama publicly supported releasing the report's findings and conclusions, the administration privately pushed to keep significant parts of the summary secret, Feinstein said.

"The president said that he agreed the report should be made public, that he doesn't condone (the harsh interrogations), but it sort of ends there," Feinstein said.

She said she perceived "an incredible closeness" between Obama's chief of staff, Denis McDonough, and Brennan, "and the president and John Brennan." In negotiations with Feinstein about what parts of the summary should be censored, McDonough spoke for the White House, but there was no daylight between him and the CIA, she said.

Feinstein said both wanted to black out large chunks of the executive summary in the name of protecting sensitive information.

In the end, she prevailed on many points after her staff demonstrated that much of what the CIA wanted to withhold was already public in memoirs and government reports. But she had to accede to the CIA's demand to leave out pseudonyms that would have allowed readers to trace the role of individual CIA officers without knowing their true names. White House spokesman Ned Price said all the redactions were justified for national security reasons.

The already fraught relationship between the committee and the administration was complicated by the flap over allegations that the CIA late last year improperly snooped on investigators' computers, and by the CIA's countercharges that the investigators wrongly took documents to which they were not entitled.

The documents were part of a CIA review that Feinstein says supports the committee's assertions that torture didn't work. The review remains classified and is the subject of a public records lawsuit.

Even after an agreement on redactions was reached, Feinstein said, the executive branch made a last-ditch effort to stop the report from becoming public. And it almost worked.

On Friday, Dec. 5, just days before Feinstein was to release the summary, she said, Secretary of State John Kerry called her to warn that embassies were likely to be attacked and Americans likely to die if the details of brutal CIA interrogations were made public. A classified intelligence assessment by the Office of the Director of National Intelligence said the same thing, though Senate aides say it was based on an incorrect summary of what was in the Senate report.

"He was very concerned that it might spark an attack here or there. And I actually didn't think it would," Feinstein said. "But how could I be sure?"

Feinstein delayed the release as she considered Kerry's request that she turn the report over to the president, who could decide when the time was right to make it public.

But then James Clapper, the director of national intelligence, briefed senators in a conference call on the secret damage assessment. Most Democrats were not convinced. Sen. Mark Udall of Colorado, who had been defeated for re-election, told a local newspaper about the possibility he would make the report public on his own, by entering it into the Senate record.

No attack or act of violence has been publicly attributed to the Senate report.

"You can either cover it up," Feinstein said, "or you try to say that history must prevent this."