WASHINGTON (INP) - It seemed an innocuous, catch-up phone call. Last year Abu Ahmed al-Kuwaiti, the pseudonym for a Pakistani known to US intelligence as the main courier for Osama bin Laden, took a call from an old friend. Where have you been? inquired the friend. Weve missed you. Whats going on in your life? And what are you doing now? Kuwaitis response was vague but heavy with portent: Im back with the people I was with before. There was a pause, as if the friend knew that Kuwaitis words meant he had returned to bin Ladens inner circle, and was perhaps at the side of the al-Qaeda leader himself. The friend replied, May God facilitate. When US intelligence officials learned of this conversation, they knew they had reached a key moment in their decade-long search for al-Qaedas founder. The call led them to the unusual, high-walled compound in Abbottabad, Washington Post reported Saturday. This is where you start the movie about the hunt for bin Laden, said one US 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 that bin Laden was in the compound. The officials would speak about the collection of intelligence and White House decision making only on the condition that they not be named. US intelligence agencies had been searching for Kuwaiti for at least four years; the call with the friend gave them the number of the couriers 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. US officials were stunned to realise 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 scrutinised 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 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. 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 analyses satellite imagery, to determine the pacers height. The agency said the mans height was somewhere between 5-foot-8 and 6-foot-8, according to one official. Another official said the agency provided a narrower range for the pacers height, but the estimate was still of limited reliability because of the lack of information about the size of the buildings windows or the thickness of the compounds 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, officials said. 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. Obama and his advisers debated the options, officials 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 vaporised 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.