The long trail to Osama bin Laden began with the arrest in 2005 of a senior al-Qaeda operative known as Abu Faraj al-Libbi. He had spent the previous two years as bin Laden’s “official messenger” to others within the terrorist group, according to military documents. And now, Libbi was being turned over to the CIA.

The resulting interrogation provided critical early intelligence that allowed the agency to begin unraveling bin Laden’s courier network and eventually to discover his hideout in the garrison city of Abbottabad, Pakistan, U.S. officials said.

Libbi was among the detainees who “flagged for us individuals who might have been providing direct support to bin Laden,” said a senior administration official, speaking on the condition of anonymity.

These supporters included a courier whose movements ultimately led U.S. Special Operations forces to bin Laden’s compound on Sunday, the official said. The official would not name the courier but said he was a protege of Khalid Sheik Mohammed — the self-avowed architect of the Sept. 11, 2001, attacks — and a “trusted assistant” of Libbi.

Libbi was held at CIA “black sites,” or secret prisons, where he was subjected to harsh questioning, which the George W. Bush administration called “enhanced interrogation techniques.” Bush administration officials, including former vice president Richard B. Cheney, said Monday that the controversial CIA interrogation program, which Obama shut down, may have played a role in finding bin Laden.

“I would assume that the enhanced interrogation program that we put in place produced some of the results that led to bin Laden’s ultimate capture,” said Cheney, speaking on Fox News. “But I’m still — I need to know more, and I’m sure we’ll all learn more over the course of the next few days.”

Bin Laden strictly avoided phone or e-mail communications for fear that they would be intercepted. Instead, he relied exclusively on couriers, a system that enabled him to avoid detection for nearly a decade after the Sept. 11 attacks. Bush administration officials said that toward the end of their tenure, they sensed that the terrorist courier network was critical to bin Laden’s capture.

“We had cued onto the notion of the courier network being the holy grail and the pathway to burrow into al-Qaeda’s senior leadership,” said Juan Zarate, who served as deputy national security adviser for terrorism issues in Bush’s second term. “We had enough information to suggest there were a couple of trusted couriers potentially being used by bin Laden.”

Still, as tantalizing as the tip was, it seemed far from iron-clad. “The reason we were all saying the trail had gone cold was that it had not borne fruit yet,” Zarate said. “We hadn’t had a sniff of bin Laden, really, since Tora Bora,” he added, in a reference to the Afghan cave complex.

An Obama administration official said that detainees described a particular man, who used a pseudonym, “as one of the few al-Qaeda couriers trusted by bin Laden, [and] indicated he might be living with or protecting Osama bin Laden.”

The official declined to further identify the courier, who was ultimately tracked down along with his brother at the compound where bin Laden was killed. U.S. officials said two of the three men killed at the compound in addition to bin Laden were couriers.

“We’ve been staring at the compound for months trying to figure out for sure whether we had enough to go with,” one official told reporters after the raid. Operatives have “been working this target for years, years, years. They finally found the guy who led to the guy who led to the guy who led to the guy, and this is it.”

Mike Rogers (R-Mich.), chairman of the House intelligence committee, praised the CIA and “the folks at the NSA who got just little snippets, little tidbits and put it all in one place and started drawing that noose.”

“They would use cutouts to cutouts, meaning they had people who met people they didn’t know to deliver a message to another person they didn’t know [and it] eventually worked its way back to Osama bin Laden,” said Rogers, describing the rings of security around the al-Qaeda leader.

John McLaughlin, who was deputy CIA director at the time of the Sept. 11 attacks, said that in any manhunt, counterterrorism officials take the smallest clue to leverage more information until they exhaust a line of inquiry or find their target. Bin Laden “was one of the highest priorities in the CIA’s menu, and there was never a question of giving up,” he said.

At the moment of his 2005 capture, Libbi was waiting at a storefront drop point in Mardan, Pakistan, for a bin Laden “designated courier” known as Maulawi Abd al-Khaliq Jan, according to his “detainee assessment,” which was written by intelligence analysts at Guantanamo Bay, Cuba. The assessment, and those of most other detainees at the military prison, were released by the anti-secrecy group WikiLeaks.

Staff writer Ellen Nakashima and staff researcher Julie Tate contributed to this report.