Trail leading to bin Laden began with his trusted courier

(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.