Washington, DC (CBS) - A hotly disputed Senate report on torture concludes that waterboarding and other harsh interrogation methods provided no key evidence in the hunt for Osama bin Laden, according to congressional aides and outside experts familiar with the investigation.
The CIA still disputes that conclusion.
From the moment of bin Laden's death almost three years ago in what was America's biggest counterterrorism success, former Bush administration and some senior CIA officials have cited the evidence trail leading to the al-Qaeda mastermind's compound in Pakistan as vindicating the "enhanced interrogation techniques" they authorized after the Sept. 11, 2001, attacks.
But Democratic and some Republican senators have disputed that account. They described simulated drownings, sleep deprivation and other such practices as cruel and ineffective. With the release edging closer for the Senate Intelligence Committee's report on interrogations, renditions and detentions, they hope to make a persuasive case.
The report, congressional aides and outside experts said, examines the treatment of several high-level terror detainees and the information they provided on bin Laden. The aides and people briefed on the report spoke on condition of anonymity because they were not authorized to speak publicly about the confidential document.
The most high-profile detainee linked to the bin Laden investigation was Khalid Sheikh Mohammed, whom the CIA waterboarded 183 times. Mohammed, intelligence officials have noted, confirmed after his 2003 capture that he knew an important al-Qaeda courier with the nom de guerre Abu Ahmed al-Kuwaiti.
But the report concludes that such information wasn't critical, according to the aides. Mohammed only discussed al-Kuwaiti months after being waterboarded, while he was under standard interrogation, they said. And Mohammed neither acknowledged al-Kuwaiti's significance nor provided interrogators with the courier's real name.
The debate over how investigators put the pieces together is significant because years later, the courier led U.S. intelligence to the sleepy Pakistani military town of Abbottabad. There, Navy SEALs killed bin Laden in a secret mission.
The CIA also has pointed to the value of information provided by senior al-Qaeda operative Abu Faraj al-Libi, who was captured in 2005 and held at a secret prison.
U.S. officials have described how al-Libi made up a name for a trusted courier and denied knowing al-Kuwaiti. Al-Libi, they said, was so adamant and unbelievable in his denial that the CIA took it as confirmation he and Mohammed were protecting the courier.
But the report concludes evidence gathered from al-Libi wasn't significant either, the aides said.
Essentially, they argued, Mohammed, al-Libi and others subjected to harsh treatment confirmed only what investigators already knew about the courier. And when they denied the courier's significance or provided misleading information, investigators would only have considered that significant if they already presumed the courier's importance.
The aides did not address information provided by yet another al-Qaeda operative, Hassan Ghul, captured in Iraq in 2004. Intelligence officials have described Ghul as the true linchpin of the bin Laden investigation after he identified al-Kuwaiti as a critical courier.
In a 2012 news release, Sen. Dianne Feinstein, D-Calif., the head of the Senate Intelligence Committee, and Sen. Carl Levin, D-Mich., acknowledged an unidentified "third detainee" had provided relevant information on the courier. But they said he did so the day before he was subjected to harsh CIA interrogation. "This information will be detailed in the Intelligence committee's report," the senators said at the time.
In any case, it still took the CIA years to learn al-Kuwaiti's real identity: Sheikh Abu Ahmed, a Pakistani man born in Kuwait. How the U.S. learned of Ahmed's name is still unclear.
Without providing full details, aides said the Senate report illustrates the importance of the National Security Agency's efforts overseas. Intelligence officials have previously described how in the years, when the CIA couldn't find where bin Laden's courier was, NSA eavesdroppers came up with nothing until 2010 - when Ahmed had a telephone conversation with someone monitored by U.S. intelligence.
At that point, U.S. intelligence was able to follow Ahmed to bin Laden's hideout.
Feinstein and other senators have spoken only vaguely of the contents of the classified review.
But they have made references to the divergence between their understanding of how the bin Laden operation came together and assertions of former CIA and Bush administration officials who have defended harsh interrogations.
Responding to former CIA deputy director Jose Rodriguez's argument that Mohammed and al-Libi provided the "lead information" on the bin Laden operation, Feinstein and Levin said, "The original lead information had no connection to CIA detainees."
They rejected former CIA Director Michael Hayden's claim that evidence on the couriers began with interrogations at black sites and Attorney General Michael Mukasey's declaration that intelligence leading to bin Laden began with Mohammed.
The facts, they said, show that the CIA learned of the courier, his true name and location "through means unrelated to the CIA detention and interrogation program." They have cited a "wide variety of intelligence sources and methods."
Terror suspects who were waterboarded "provided no new information about the courier" and offered no indication of where bin Laden was hiding, the senators said.
Feinstein will push to release a summary of the intelligence committee's review later this week, starting a declassification process that could take several months before any documents are made public.
Senate investigators and CIA officials already are locked in a simmering dispute over competing claims of wrongdoing in the congressional investigation. Feinstein accuses the agency of improperly monitoring the computer use of Senate staffers and deleting files, undermining the Constitution's separation of powers. The CIA says the intelligence panel illegally accessed certain documents. The Justice Department is reviewing criminal complaints against both sides.
Aides said Levin and Sen. John McCain, R-Ariz., who himself was tortured as a prisoner war in Vietnam more than four decades ago, are among those pushing hard to ensure the investigation's findings related to the bin Laden pursuit and CIA interrogations are made public.
They and Feinstein were among Congress' critics of how the hunt was portrayed in the film "Zero Dark Thirty," which they said was fictional.