It's been more than two years since they last picked up the trail of the world's most wanted man, says a top Pakistani intelligence officer in a hasty meeting at a secret rendezvous point.
"At one stage in early 2003 we thought we were quite close to him," murmurs the anti-terrorism official, whose high rank and sensitive work meant that he would only speak if his identity was not revealed.
"But a few hours before the operation could start in the border terrain near Afghanistan, he moved out."
Since then, nothing. Osama bin Laden's vanishing act continues to baffle the world's biggest military power and its allies, four years after the September 11, 2001, attacks on the United States.
While the smoke was still rising from the rubble of the Twin Towers, US President George W. Bush vowed to track down the Al-Qaeda chief dead or alive. Weeks later, US-led forces invaded Afghanistan to topple the Taliban, the hardline Islamic regime that harboured bin Laden.
Many officials believe he was trapped late in 2001 by the ferocious US bombing of the Tora Bora cave complex in eastern Afghanistan, but he managed to slip out of the region, possibly across the porous border into Pakistan.
Thousands of people have since been arrested around the world, many confined to Guantanamo Bay without charge, but none has been able to provide the crucial nugget of information.
The Saudi, who has acquired poster-boy status in parts of the Muslim world, is also credited with inspiring Al-Qaeda offshoots to carry out new atrocities, including the March 2004 Madrid bombings and this year's July 7 suicide attacks in London.
Also wanted by the United States is bin Laden's right-hand-man Ayman al-Zawahiri, who appeared on a new video aired 10 days before the anniversary of the September 11 attacks. The tape also showed one of the London bombers.
Even a 25-million-dollar reward offered by the United States has yielded no visible results, while Pakistan's President General Pervez Musharraf said late last year that the trail had "gone cold".
Inevitably, the focus remains on Pakistan, an enthusiastic ally in Bush's "war on terror" and the site of almost all the key Al-Qaeda captures since 9/11.
"We have been saying previously and we still maintain that we are not into manhunt," Pakistan's chief military spokesman, Major General Shaukat Sultan, told AFP at army headquarters in Rawalpindi, near Islamabad.
"If your sole objective was capture of OBL, yes we remain where we were but ... in Pakistan that is not our sole objective. Our objective is to root out terrorism and we have progressed quite a lot."
Two years ago, at around the time of the bin Laden sighting reported by the intelligence official, Pakistan captured the self-proclaimed mastermind of the September 11 attacks, Khalid Sheikh Mohammed.
Earlier this year it snared Mohammed's alleged successor as Al-Qaeda's third in command, Abu Faraj al-Libbi, using intelligence agents disguised as women wearing burqas.
Sultan admits the captures of these men, both of whom have been handed over to the United States, have not provided the hoped for leads to bin Laden.
But he adds that apart from a menacing video tape delivered to a satellite TV channel in Islamabad just before the 2004 US elections, bin Laden's silence shows the authorities there are having some success.
One key Al-Qaeda suspect revealed under interrogation that bin Laden was using couriers travelling on foot or horseback instead of communicating by satellite telephone or the Internet, Sultan said.
"It generally takes them about two months to get the message across and get its response," said Sultan. "This is the time involved in that, so one can make rough guess about where would the man be."
US Central Intelligence Agency director Porter Goss in June said he had an "excellent idea" of where bin Laden was hiding -- though he didn't say where.
Meanwhile officials and analysts say there are signs that Washington is again stepping up its efforts to track down bin Laden, after two years of concentrating on the bloody, Al-Qaeda-linked insurgency in Iraq.
Elite US Delta Force and Navy SEAL units have just started to come back to Afghanistan after tours of duty in Iraq, a US counter-terrorism official based in Washington told AFP, and are spearheading the hunt.
"Several special forces teams are stationed right at the border and use special sensors along the roads to pick up sound and vibration from the movement of cars," he said.
In a potentially controversial move, US agents have also been secretly pursuing what the US calls "Operation Enduring Freedom" beyond Afghanistan itself, he said.
"Some cross-border reconnaissance raids by intelligence agents are taking place from Afghanistan into South and North Waziristan, and all the way up to Bajaur and the Northern Areas," the official added, referring to three of Pakistan's wild tribal zones plus its most mountainous region.
But even if the opportunity that passed so fleetingly in 2003 came up again, it is a widely held view among security officials that neither bin Laden nor al Zawahiri would let themselves be captured alive.
Security sources say the pair are reported to wear explosive belts and their huge retinue of Arab bodyguards have orders to kill them if it is impossible to escape.