WASHINGTON — It seemed an innocuous, catch-up phone call. Last year Abu Ahmed al-Kuwaiti, the pseudonym for a Pakistani known to U.S. intelligence as the main courier for Osama bin Laden, took a call from an old friend.
Where have you been? asked the friend. What's going on in your life? And what are you doing now?
Kuwaiti's response was vague but heavy with portent: "I'm back with the people I was with before."
There was a pause, as if the friend knew that Kuwaiti's words meant he had returned to bin Laden's inner circle and was perhaps at the side of the al-Qaida leader himself.
The friend replied, "May God facilitate."
When U.S. intelligence officials learned of this exchange, they knew they had reached a key moment in their decadelong search for al-Qaida's founder. The call led them to the unusual, high-walled compound in Abbottabad, a city 35 miles north of Pakistan's capital.
This story from the Washington Post relied on officials who would speak about the collection of intelligence and White House decision-making only on the condition that they not be named.
The cellphone exchange and several other pieces of information gave President Barack Obama the confidence to launch a politically risky mission, the Post reported, to capture or kill bin Laden, a decision he took despite dissension among his key national security advisers and varying estimates of the likelihood that bin Laden was in the compound.
U.S. intelligence agencies had been hunting for Kuwaiti for at least four years; the call with the friend gave them the number of the courier's cellphone. Using a vast number of human and technical sources, they tracked Kuwaiti to the compound.
The main three-story building, which had no telephone lines or Internet service, was impenetrable to eavesdropping technology deployed by the National Security Agency.
U.S. officials were stunned to realize that whenever Kuwaiti or others left the compound to make a call, they drove some 90 minutes away before even placing a battery in a cellphone.
Turning on the phone made it susceptible to the kind of electronic surveillance that the residents of the compound clearly wished to avoid.
As intelligence officials scrutinized images of the compound, they saw that a man emerged most days to stroll the grounds of the courtyard for an hour or two.
The man walked back and forth, day after day, and soon analysts began calling him "the pacer." The imagery never provided a clear view of his face.
Intelligence officials were reluctant to bring in other means of technical or human surveillance that might offer a positive identification but that would risk detection by those in the compound.
The pacer never left the compound. His routine suggested he was not just a shut-in but almost a prisoner.
Was the pacer bin Laden? A decoy? A hoax? A setup?
Bin Laden was at least 6-foot-4, and the pacer seemed to have the gait of a tall man.
The White House asked the National Geospatial-Intelligence Agency, which provides and analyzes satellite imagery, to determine the pacer's height.
The agency said the man's height was somewhere between 5-foot-8 and 6-foot-8, according to the Post's account from one official.
The Post reported that another official said the agency provided a narrower range for the pacer's height, but the estimate was still of limited reliability because of the lack of information about the size of the building's windows or the thickness of the compound's walls, which would have served as reference points.
In one White House meeting, CIA director Leon Panetta told Obama and other top national security officials that the general rule in gathering intelligence was to keep going until a target such as the Abbottabad compound ran dry.
Panetta said that point had been reached, arguing that those tracking the compound were seeing the pacer nearly every day but could not conclude with certainty that it was bin Laden, according to the officials in the Post report. Panetta noted that there was no signals intelligence available and contended that it was too risky to send in a human spy or move any closer with electronic devices. The agency established a safe house in Abbottabad for a small team that monitored the compound in the months leading up to the raid.
Obama and his advisers debated the options, the Post said.
One option was to fire a missile from a Predator or Reaper aerial drone. Such a strike would be low-risk, but if the result was a direct hit, the pacer might be vaporized and officials would never be certain they had killed bin Laden.
If the drone attack missed, as had happened in attacks on high-value targets, bin Laden or whoever was living in the compound would flee and the United States would have to start the hunt from scratch.
Panetta designated Navy Vice Adm. William McRaven, who had headed the Joint Special Operations Command for nearly three years, to devise a boots-on-the ground plan for the special forces that became known as "the McRaven option."
His decision to assign the operation to the Navy SEALs, a special operations unit with extensive experience in raids on high-value targets, was critical. SEALs have a tradition of moving in and out fast, often killing everyone they encounter at a target site. Most members of the SEAL team in the bin Laden raid had been deployed to war zones a dozen or more times.
Specific orders were issued to the SEALs not to shoot women or children unless they were clearly threatening or had weapons. (During the mission, one woman was killed and a wife of bin Laden was shot in the leg.)
The Post quoted one official as saying Bin Laden was to be captured if he conspicuously surrendered.
Several assessments concluded there was a 60 to 80 percent chance that bin Laden was in the compound. Michael Leiter, the head of the National Counterterrorism Center, was much more conservative. During one White House meeting, he put the probability at about 40 percent.
When a participant suggested that was a low chance of success, Leiter said, "Yes, but what we've got is 38 percent better than we have ever had before."
According to the Post, officials said Obama's national security advisers were not unanimous in recommending he go ahead with the McRaven option.
The president approved the raid at 8:20 a.m. April 29.
During the assault, one of the Blackhawk helicopters stalled, but the pilot was able to land safely.
The hard landing, which disabled the helicopter, forced the SEALs to abandon a plan to have one team rope down from a Blackhawk and come into the main building from the roof. Instead, both teams assaulted the compound from the ground.
Bin Laden was shot and killed. Later, White House spokesman Jay Carney said bin Laden resisted in some form. He and others have declined to specify the exact nature of his alleged resistance.
SEALs scooped up dozens of thumb drives and several computer hard drives that are now being scrutinized for information about al-Qaida, especially an address, location or cellphone number for Ayman al-Zawahri, bin Laden's second in command.
In the White House Situation Room on Sunday night, the president and his national security team watched a soundless video feed of the raid.
When bin Laden's corpse was laid out, one of the Navy SEALs was asked to stretch out next to it to compare heights. The SEAL was 6 feet tall. The body was several inches taller.
After the information was relayed to Obama, he turned to his advisers and said: "We donated a $60 million helicopter to this operation. Could we not afford to buy a tape measure?"