The United States had started wiretapping a phone number associated with Mr. Kuwaiti by late 2001, and as early as 2002, the C.I.A. had obtained from other sources — including reports from allies based on detainees in their custody — the courier’s alias and the fact that he was one of Bin Laden’s few close associates and “traveled frequently” to meet with him. It also had data on his age, physical appearance and family connections, as well as a recording of his voice — all of which proved crucial to finding him.

It was in 2004 that the C.I.A. came to realize that it should focus on finding Mr. Kuwaiti as part of the hunt for Bin Laden, after it interrogated a Qaeda operative, Hassan Ghul, who had been captured in Iraqi Kurdistan. The report concludes that Mr. Ghul provided “the most accurate” intelligence that the agency produced about Mr. Kuwaiti’s role and ties to Bin Laden.

Image The Senate Intelligence Committee report discredits the notion that the C.I.A. would not have found Osama bin Laden if it had not tortured detainees. Credit... Associated Press

But the report emphasizes that Mr. Ghul provided all the important information about the courier before he was subjected to any torture techniques and spoke freely to his interrogators. During that two-day period in January 2004, it said, the C.I.A. produced 21 intelligence reports from Mr. Ghul, who one officer said “sang like a tweetie bird.”

“He opened up right away and was cooperative from the outset,” the officer added.

In those initial interrogations, Mr. Ghul portrayed Mr. Kuwaiti as Bin Laden’s “closest assistant” and said he was always with him, identifying him as a likely courier who ran messages between Bin Laden and other leaders of Al Qaeda. He listed him as one of three people most likely to be with Bin Laden, who he speculated was living in a house in Pakistan, with Mr. Kuwaiti handling his needs.

Nevertheless, the C.I.A. then decided to torture Mr. Ghul to see if he would say more. He was transferred to a “black site” prison, where he was shaved, placed in a “hanging” stress position, and subjected to 59 hours of sleep deprivation, after which he began hallucinating; his back and abdomen began spasming; his arms, legs and feet began experiencing “mild paralysis”; and he began having “premature” heart beats. During and after that treatment, he provided “no actionable threat information” that resulted in the capture of any leaders of Al Qaeda, the report said.