Years of dogged intelligence work led to Sunday's raid that killed Osama bin Laden, with US spy agencies chasing the trail of a trusted courier to the al Qaeda chief, officials said.
The operation took less than 40 minutes but was the result of a methodical effort, as US intelligence agencies had bin Laden in their sights for months while administration officials planned a risky manhunt inside Pakistan.
The roots of the raid date back four years ago, when intelligence agencies at last managed to identify bin Laden's personal courier in a long-awaited breakthrough, a senior US official told reporters.
Terror suspects in interrogations had "identified this man as one of the few al Qaeda couriers trusted by bin Laden," said the official, who spoke on condition of anonymity.
"They indicated he might be living with and protecting bin Laden. But for years, we were unable to identify his true name, or his location."
Then about two years ago, the spy services "identified areas where the courier and his brother operated," the official said.
"Still, we were unable to pinpoint exactly where they lived due to extensive operational security on their part. The fact that they were being so careful reinforced our belief that we were on the right track," he said.
"Then in August 2010, we found their residence."
The compound, in the affluent suburbs of Abbottabad about 50 kilometres (30 miles) northwest of the Pakistani capital Islamabad, immediately grabbed the attention of intelligence analysts, the official said.
"When we saw the compound where the brothers lived, we were shocked by what we saw," the official said.
The compound, with two security gates, dwarfed other homes in the area and although valued at one million dollars, had no telephone or Internet service.
"The physical security measures of the compound are extraordinary. It has 12 to 18-foot walls topped with barbed wire. Internal walls section off different portions of the compound to provide extra privacy," the official said.
"Intelligence analysts concluded this compound was custom built to hide someone of significance," he said.
Washington soon learned that apart from the courier and his brother, a third family lived there, one that appeard to match up with the profile of bin Laden's family members -- including his youngest wife.
"Everything we saw, the extremely elaborate operational security, the brothers' background and their behavior, and the location and the design of the compound itself, was perfectly consistent with what our experts expected bin Laden's hideout to look like," the official said.
The intelligence agencies got confirmation from other sources that bin Laden was likely at the compound but took pains to check and question their information, he said.
"No other candidate fit the bill as well as bin Laden did."
By February, the intelligence services were convinced they had found bin Laden, and the White House began preparations for a raid deep inside Pakistan, officials said.
Key national security deputies planned the operation "for months and briefed the president regularly," another administration source said.
"The high walls, the security features, suburban location and proximity to Islamabad made this an especially dangerous mission," the official said.
Starting in March, President Barack Obama held a series of meetings with his national security team on possible options.
Only a small number of officials in the administration knew what was in the works and Washington chose not to inform Pakistan.
"We shared our intelligence on this bin Laden compound with no other country, including Pakistan. That was for one reason and one reason alone: We believed it was essential to the security of the operation and our personnel," a third US official said.
On Friday at 8.20am Washington time, Obama made the decision to go ahead with the helicopter raid, before setting off for a trip to Alabama, officials said.
In a "surgical raid" on Sunday that lasted less than 40 minutes, US helicopters delivered a "small team" to the heavily fortified compound, another official said.
The assault team killed Osama bin Laden in a firefight as well as the courier, the courier's brother, and an adult son of the al Qaeda leader, another official said.
A woman was also killed in the operation and US officials said she had been used as a shield by one of Bin Laden's fighters.
The US team, which suffered no serious casualties, presumably included members of elite special operations forces but administration officials offered few details of the raid.
One helicopter in the raid went down due to a "mechanical failure," the official said.
The aircraft was blown up by the crew and "the assault force and crew members boarded the remaining aircraft to exit the compound," he said.
By late Sunday night, the White House alerted news outlets that the US president would address the nation on television on a matter of national security.