WASHINGTON — Helicopters descended out of darkness on the most important counterterrorism mission in U.S. history. It was an operation so secret, only a select few U.S. officials knew what was about to happen.

The location was a fortified compound in an affluent Pakistani town two hours outside Islamabad. The target was Osama bin Laden.

Intelligence officials discovered the compound in August while monitoring an al-Qaida courier. The CIA had been hunting that courier for years, ever since detainees told interrogators that the courier was so trusted by bin Laden that he might very well be living with the al-Qaida leader.

Nestled in an affluent neighborhood, the compound was surrounded by walls as high as 18 feet, topped with barbed wire. Two security gates guarded the only way in. A third-floor terrace was shielded by a seven-foot privacy wall. No phone lines or Internet cables ran to the property. The residents burned their garbage rather than put it out for collection. Intelligence officials believed the million-dollar compound was built five years ago to protect a major terrorist figure. The question was, who?