(CNN) -- Osama bin Laden's death marked the end of an era for U.S. investigators, who searched remote villages and mountain caves in a far-reaching manhunt for the al Qaeda leader.
Their search, which lasted more than a decade, ended early Monday in Pakistan (Sunday night in the U.S.), when a small U.S. team raided a heavily guarded hideout in Pakistan and killed him.
The key break in the case came in August, when senior Obama administration officials say U.S. intelligence homed in on a $1 million compound in an affluent area north of Islamabad where one of bin Laden's couriers lived.
But the trail that eventually led the United States to bin Laden began years ago, the officials said.
After years of thwarted searches and dead ends, finding the elusive bin Laden had become "America's most vexing intelligence problem," one senior Obama administration official told reporters in a background briefing in Washington early Monday.
A new lead emerged when post-9/11 detainees gave investigators a glimpse into the al Qaeda chief's inner circle, the official said. During questioning, detainees repeatedly mentioned the nickname of a man they said was one of the few couriers bin Laden trusted.
That was the beginning of what President Barack Obama's top counterterrorism adviser described as a painstaking process.
"From the nickname, we tried to find out his real name," a senior U.S. official familiar with the operation said. "It was classic espionage and intel work."
Investigators knew the courier -- a protege of 9/11 mastermind Khalid Sheikh Mohammed -- was "important," because a number of detainees held out on providing information about him, the senior U.S. official said.
When interrogators pushed Mohammed on the courier's identity, he "lied to protect his protege," the official said. "We knew he was lying, because we already knew (the courier) was a KSM protege," the official said. In fact, other detainees indicated that the courier could have been working for bin Laden, the official added.
While Mohammed held back on information, that in itself made intelligence personnel more interested in the courier because they knew Mohammed was lying, the official said.
U.S. intelligence uncovered the courier's identity four years ago "from a different part of the world," the senior U.S. official said. He declined to say where.
It took two more years to identify where the courier and his brother operated in Pakistan.
Then, "extensive operational security" kept investigators from pinpointing where they lived, the senior administration official said.
"The fact that they were being so careful reinforced our belief that we were on the right track," the official said.
Spotting the courier on a street in Pakistan gave them an important clue, the senior U.S. official familiar with the operation said.
"We couldn't trail him, so we had to set up an elaborate surveillance effort ... That finally tracked him back to that compound," the senior U.S. official said.
In August 2010, officials said, U.S. intelligence drilled down to the home where the two brothers lived with their families, located in Abbottabad, Pakistan -- about 35 miles north of Islamabad.
"When we saw the compound where the brothers lived, we were shocked by what we saw -- an extraordinarily unique compound," the senior administration official said. "The compound sits on a large plot of land in an area that was relatively secluded when it was built. It is roughly eight times larger than the other homes in the area."
The senior Obama administration official said several things about the compound immediately stood out: Residents burned their trash rather than leaving it out for collection, there was no telephone or internet service and the three-story main structure had few windows facing outside. In addition, 12- to 18-foot walls topped with barbed wire surrounded the compound.
Intelligence analysts concluded the compound was "custom-built to hide someone of significance," the official said, and they noted that the courier and his brother had no discernible source of wealth to live at such a property.
"Everything we saw ... was perfectly consistent with what our experts expected bin Laden's hideout to look like," the official said.
As more details emerged, CIA Director Leon Panetta "began to hammer folks very hard" about who lived inside, the senior U.S. official with knowledge of the operation said.
"They counted people and told Panetta who was sleeping where," the official said.
Intelligence soon revealed that a third family lived at the compound, the senior administration official said, "one whose size and whose makeup matched the bin Laden family members that we believed most likely to be with Osama bin Laden."
Even after analysts used multiple methods to analyze "every angle" of the information, "no other candidate fit the bill as well as bin Laden did," the official said.
U.S. officials did not release the courier's identity or specify how they found the home. A senior Pakistani intelligence official mentioned electronic and phone tracking, and said the U.S. intelligence on bin Laden's whereabouts was primarily based on Pakistani information passed along to the Americans.
Following bin Laden's couriers was "the only way" authorities ever could have tracked down the al Qaeda leader, CNN National Security Analyst Peter Bergen said.
"He was in a Catch-22. He had to communicate a little bit, and the only way he could do that without electronic communication, which could be intercepted, was through couriers," Bergen said.
Senior administration officials said the CIA informed Obama in September that assessments indicated bin Laden could be in a compound in Pakistan. By mid-February, the intelligence was considered strong enough to begin considering action pledged by Obama when bin Laden's whereabouts had been determined.
To discuss that intelligence and develop a plan, Obama chaired five National Security Council meetings from mid-March until late April, with the last two on April 19 and April 28 -- last Thursday.
The next day, Obama gave the order for the mission, officials said.
Even so, officials were not certain they would find bin Laden, said John Brennan, Obama's top counterterrorism adviser.
"There was nothing that confirmed bin Laden was at that compound," he said, noting that Obama's decision to act after weighing the evidence was one of the "gutsiest calls of any president in memory."
In fact, the intelligence community never laid its collective eyes on bin Laden in or around the compound before the raid, according to a U.S. official with knowledge of the intelligence build-up.
In his remarks Monday, Brennan referred to the intelligence as "circumstantially" pointing to bin Laden being in the compound.
"I think what this operation demonstrates is that there are some very, very good people who have been following bin Laden for many, many years. They have been very persistent," Brennan said.
"They have pulled on every thread. And as a result of that diligence and their analytic capabilities, they were able to track this and continue to build a body of evidence that suggested, circumstantially, that bin Laden was at that compound."
At 8:20 a.m. ET Friday, before leaving on a trip to tour tornado devastation in Alabama, Obama made the decision to undertake the operation, a senior administration official said.
Late Sunday night in a White House address to the American people, Obama described the daring raid by a small U.S. team that killed bin Laden.
One U.S. government official told CNN the operation that killed the founder and leader of al Qaeda was designed to do just that, not to take him alive. But another senior U.S. official told CNN the operation included instructions to arrest bin Laden alive if he had surrendered -- however, no one involved expected that he would surrender.
Authorities have not determined how long the three-story home has been bin Laden's hideaway.
"The compound had been in existence for roughly five years, but we don't know how long bin Laden lived there," a senior administration official said at the Monday.
But they were unequivocal in their declarations of his death.
On the web page containing the FBI's most-wanted list, a clear message was spelled out in bold letters beneath bin Laden's picture: "Deceased."
"Decapitating the head of the snake ... will have important reverberations throughout the area and the al Qaeda network in that area. This is something that we've been after for 15 years," Brennan said.
In an e-mail to CIA staff Monday, Panetta praised the "highly complex, innovative and forward-leaning clandestine operations" that he said led them to bin Laden.
"We have rid the world of the most infamous terrorist of our time," he wrote.
CNN's Gloria Borger, Barbara Starr, Adam Levine, Nick Paton Walsh, Pam Benson and Suzanne Kelly contributed to this report.