Pike says it would have been exceedingly careless for the courier, identified as Sheikh Abu Ahmed, to use a phone at all, considering al-Qaida's well-known avoidance of telephones. The NSA was reportedly monitoring not Ahmed but whoever it was who called him, and stumbling across bin Laden's courier was a stroke of good luck for the government.
Once American eyes were trained on the Abbottabad compound, the lack of a phone line or Internet access on the property would have challenged NSA's eavesdroppers. But Pike theorized about other spying methods that may have been used.
According to one story Pike heard from his intelligence sources, American officials concluded there was one person at the compound who never ventured outside — presumably bin Laden. Pike described how he thinks agents might have reached that conclusion, saying it sounds like the work of the Special Collection Service, a joint NSA-CIA surveillance operation.
"I would do that by having a Special Collection Service team get an apartment a mile away and start shining laser beams on all the windows," he said. "The voice noise on the inside of the room is going to cause window glass to vibrate. A laser beam illuminates the window glass and detects the vibration."
By analyzing the vibrations, he said, analysts could tease out the number of distinct voices. "I can do voice identification to count how many people I'm listening to. If I count 22 people inside the building and 21 outside the building, I know I've got somebody who never goes outdoors."
The New York Times reported that for months the CIA had used a nearby rented house to watch and photograph people at the compound.
The newspaper said CIA agents used cameras with telephoto lenses and infrared imaging equipment. It also said they used sensitive eavesdropping equipment in an attempt to detect voices inside the building and to track any cellphone calls.
(Possibly contradicting the story Pike heard, the surveillance team reportedly saw a man take regular walks in the compound's courtyard, though they could not confirm that was bin Laden.)
The Times also reported that a satellite used radar to search for possible escape tunnels. Such imagery is supplied by the National Geospatial-Intelligence Agency, known as the NGA.
Ruppersberger credited the NGA with providing a wealth of images and other clues that helped military planners prepare for the helicopter assault. As he put it, "You had to know exactly where you were, what you were doing, how many stairs you had to climb, how many people were in there."
"To think we could go in there in 45 minutes, including blowing up one of our own helicopters — it's amazing," he said. "Pakistan, by the time they realized something was happening and were getting their jets up, we were out of there."
Parachini said the NGA "probably used everything" in its arsenal, including space imagery, photos taken on the ground by individuals and various types of aircraft carrying sensors, perhaps including unmanned drones.
And while the courier's intercepted phone call could have provided a breakthrough, he is reluctant to label anything a "silver bullet." Rather, he thinks "there was an accumulation of insights over time."
"I realize that's not as dramatic," he said, "but there's a lot of drudgery in putting lots of different things together and understanding how do these fit together."
For the intelligence community, Parachini said, "this has got to be a moment of reward, and a justifiable one." But the hunt for bin Laden has gone on so long that it has left longtime members of the intelligence community feeling temporarily at a loss.
"There was almost a moment of jubilation, but then on Tuesday, what am I focusing on now?" he said. "There's a little bit of a 'now what'? The obvious answer is there are other key leaders out there, and there will be other key leaders who emerge."
One unknown, he said, is whether bin Laden's death will sap the energy of the extremist movement that he inspired around the world.
Ruppersberger said there is no risk of complacency on the part of the U.S. government.
"Now is the time more than ever to keep pressure on al-Qaida," he said.
His top concern now, he said, is the American-born cleric, Anwar al-Awlaki. The 40-year-old served as an imam in Northern Virginia before moving to Yemen, where he is reportedly recruiting and training Islamist radicals to attack the United States.
"He's just as dangerous as bin Laden," Ruppersberger said.
Now NSA analysts are busy mining the documents and computer drives scooped up at the bin Laden compound in Pakistan.
"We want to cripple al-Qaida," Ruppersberger said. "Hopefully that information will show who the leaders are who are still active. Are there new leaders we haven't identified? Are there planned attacks in the future?"
By Thursday night, news reports said there was evidence of a preliminary al-Qaida plot to target American railways, possibly intended to coincide with the 10th anniversary of the 2001 terrorist attack that claimed nearly 3,000 American lives.