A U.S. Senate investigation concludes 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 a still-secret, 6,200-page report.

The CIA disputes the conclusion, which could deepen the worst rift in years between lawmakers and the CIA, and already is locked with the Senate intelligence committee in an acrimonious fight amid duelling charges of snooping and competing criminal referrals to the Justice Department. Americans may soon get the chance to decide, with the congressional panel planning to vote Thursday to demand a summary of its review be declassified.

From the moment of bin Laden's death almost three years ago, former Bush administration figures and top CIA officials have cited the evidence trail leading to the al-Qaeda mastermind's walled Pakistani compound as vindication of the "enhanced interrogation techniques" they authorized after the Sept. 11, 2001, attacks.

But Democratic and some Republican senators have called that account misleading, saying simulated drownings known as waterboarding, sleep deprivation and other such practices were cruel and ineffective.

The intelligence committee's report, congressional aides and outside experts said, backs up that case after examining the treatment of several high-level terror detainees and the information they provided on bin Laden. The aides and people briefed on the report demanded anonymity because they weren't authorized to speak publicly about the confidential document.

Finding courier key to search for bin Laden

The most high-profile detainee linked to the bin Laden investigation was Khalid Sheikh Mohammed, the accused 9/11 mastermind who was 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.

9/11 mastermind Khalid Sheikh Mohammed, shown here during his arrest on March 1, 2003, was reportedly waterboarded by the CIA 183 times. (U.S.News & World Report/Reuters)

The Senate report concludes 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, in May 2011, Navy commandos 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 run by the agency.

In previous accounts, U.S. officials have described how al-Libi made up a name for a trusted courier and denied knowing al-Kuwaiti. Al-Libi was so adamant and unbelievable in his denial that the CIA took it as confirmation he and Mohammed were protecting the courier.

The Senate report concludes evidence gathered from al-Libi wasn't significant either, aides said.

Essentially, they argue, 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.

NSA played a role in search: senate report

In a 2012 news release, Sen. Dianne Feinstein, the head of the Senate Intelligence Committee, and Sen. Carl Levin, both Democrats, 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.

U.S. Senator Dianne Feinstein, the head of the Senate Intelligence Committee, is set to hold a vote on whether or not to release the report's 400-page summary to the public. (Jonathan Ernst/Reuters)

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.