"This is where you start the movie about the hunt for bin Laden," said one U.S. official briefed on the intelligence-gathering leading up to the raid on Osama bin Laden's compound early Monday.

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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? inquired the friend. We’ve missed you. What’s going on in your life? And what are you doing now?

Al-Kuwaiti’s response was vague: “I’m back with the people I was with before.”

There was a pause, as if the friend knew al-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.

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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, 35 miles north of Pakistan’s capital, Islamabad.

“This is where you start the movie about the hunt for bin Laden,” said one U.S. official briefed on the intelligence-gathering leading up to the raid on the compound early Monday.

The exchange and several other pieces of information, other officials said, gave President Obama the confidence to launch a politically risky mission to capture or kill bin Laden, a decision he took despite dissension among his key national-security advisers and varying estimates of the likelihood bin Laden was in the compound.

The officials would speak about the collection of intelligence and White House decision-making only on the condition they not be named.

U.S. intelligence agencies had been hunting for al-Kuwaiti, whose real name was Arshad Khan, 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 him 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 al-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 the residents of the compound clearly wished to avoid.

As intelligence officials scrutinized images of the compound, they saw 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 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 between 5-foot-8 and 6-foot-8, according to one official.

Another official said the agency provided a narrower range for the pacer’s height, but the estimate was still of limited reliability.

In one White House meeting, CIA Director Leon Panetta told Obama and other top national-security officials 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 it was bin Laden, officials said. Panetta contended it was too risky to send in a human spy or move any closer with electronic devices.

The agency had 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, officials said. Panetta designated Navy Vice Adm. William McRaven, who had headed the Joint Special Operations Command (JSOC) for nearly three years, to devise a boots-on-the-ground plan for the special-operations forces that became known as “the McRaven option.”

McRaven’s 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.

A “pattern of life” study of the compound by intelligence agencies showed about a dozen women and children periodically frequented it.

Specific orders were issued to the SEALs not to shoot the women or children unless they were clearly threatening or had weapons. (During the mission, a wife of bin Laden was shot in the leg. There were conflicting reports on whether another woman was killed in the raid.) Bin Laden was to be captured, one official said, if he “conspicuously surrendered.”

Several assessments concluded there was a 60 to 80 percent chance bin Laden was in the compound. Michael Leiter, head of the National Counterterrorism Center, was more conservative. During one meeting, he put the probability at about 40 percent.

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. Friday.

During the assault, one of the Black Hawk 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 Black Hawk and enter the main building from the roof. Instead, both teams assaulted the compound from the ground.

Obama administration officials initially said bin Laden was shot and killed because he was engaged in a firefight and resisted. Later, spokesman Jay Carney said bin Laden was not armed, but Carney insisted he resisted in some form. He and others have declined to specify the nature of the reported resistance, though there reportedly were weapons in the room where bin Laden was killed.

U.S. officials said at least three men were killed alongside bin Laden, including one of his sons, al-Kuwaiti and al-Kuwaiti’s brother.

A senior special-operations official said SEALs would avoid providing more details about the raid, to prevent the disclosure of methods central to their success. The individuals who took part in the raid, the official said, would not grant interviews and had signed nondisclosure agreements about their classified work.

SEALs scooped up dozens of thumb drives and several computer hard drives that are 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 late Sunday, 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?”

Evelyn M. Duffy contributed

to this report.

Material from The Associated Press is included in this report.