WASHINGTON- Water-boarding and other harsh interrogation techniques provided no key evidence in the hunt for Osama bin Laden, a new senate report into the use of torture by the CIA in the years after the September 11 attacks is expected to claim.

If confirmed, the finding in the 6,200-page senate report will directly challenge assertion by former members of the George W. Bush administration that the CIA’s so-called “enhanced interrogation” techniques were an essential tool in prosecuting the war on terror. Congressional aides familiar with the still-secret report told the Associated Press that a review of some 6 million classified documents had led them to conclude there was no benefit derived from treatment that the United Nations and rights groups say amounted to torture.

The US senate’s powerful intelligence committee is due to hold a vote on Thursday on whether to release a 400-page summary of the report, setting in motion a declassification process that could take several months before documents are made public. The findings of the report have already caused a bitter and public rift between Dianne Feinstein, the Committee’s democrat chair, and the CIA whom she has openly accused of trying to frustrate the publication of the report since its findings were approved in December 2012.

Most fundamental among its findings is expected to be that the treatment meted out to Al Qa’eda suspects in sites such as Bagram Airbase in Afghanistan, the Guantánamo Bay detention centre and “black jails” around the world did not ultimately yield critical intelligence. The successful assassination of bin Laden was seized on by former Bush administration figures and top CIA officials as vindication of the “enhanced interrogation techniques” they authorised after the Sept. 11, 2001, attacks.

That narrative was strengthened in the popular imagination by Kathryn Bigelow’s controversial film Zero Dark Thirty, a dramatic reconstruction of how bin Laden’s whereabouts were pieced together that was accused by civil rights groups of wrongly depicting torture as effective. Among the key points of contention in the report was the treatment of Khalid Sheikh Mohammed, the man accused of masterminding the September 11 attacks who was waterboarded 183 times.

Intelligence officials have cited that Mohammed had confirmed that he knew an important al-Qaida courier with the nom de guerre Abu Ahmed al-Kuwaiti who helped lead CIA investigators to the compound in Abbottabad where bin Laden was killed. However the Senate report concludes such information wasn’t critical and was obtained not when Mohammed was being water-boarded, but under standard interrogation months later, the unnamed aides told the Associated Press.

The CIA also has pointed to the value of information provided by senior al-Qaida 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-Qaida 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, Ms 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.

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.