Far above the fort, spy satellites and aircraft search the skies for a stray electronic signal - a coded radio message, a satellite phone call - that might lead them to the lair of Osama bin Laden. On the ground, Special Forces operatives, sometimes bearded and dressed in civilian clothes, gather intelligence that might point to the trail of al-Qaeda's leader.
Closer to the austere outpost, heavily armed soldiers from the 10th Mountain Division patrol the rugged mountains, skirmishing with bin Laden's legacy - the Taliban and al-Qaeda remnants who appear to be regathering strength since a U.S.-led coalition ousted them from power in Afghanistan two years ago.
Though the war occupies the most manpower and resources, the psychological prize is bin Laden, whose whereabouts - along with those of former Taliban leader Mullah Mohammed Omar - remain a mystery.
"I want to get him worse than anything," said Gen. John Abizaid, the chief military officer in the U.S. Central Command, a region that includes Afghanistan, Iraq and the Middle East. "He is a huge symbol for the 3,000 Americans who died, and we need to get him, we need to get Saddam, we need to get Omar."
Military and intelligence officials believe they came close to trapping him in the Tora Bora mountains in December 2001. But he escaped, and since then the trail has gone cold.
It is a tribute to bin Laden's craftiness that the tall Arab who walks with a cane and was trained in the building trades has been able to outfox perhaps the most sophisticated and the most intense manhunt in history, even with a $25 million price on his head.
U.S. forces - who make up most of the 11,500-member coalition force in Afghanistan - remain confident that one day they will prevail, both in the hunt for bin Laden and in the war.
"I am convinced that over time, as Osama bin Laden becomes more and more isolated and as people understand the bankruptcy of his creed, we will eventually find him," Abizaid said.
After escaping the blitz of coalition forces in 2001, bin Laden is believed to have escaped into neighboring Pakistan. He has been reported to be holed up in any of a half-dozen places along the border, from the formidable Chitral Mountains in the north to the dun-colored Baluchistan deserts in the south.
U.S. officials believe the intensity of their manhunt has hampered bin Laden's command-and-control - the United States is spending $1.1 billion a month to maintain its forces in Afghanistan.
But they do not underestimate their nemesis.
"They are no dummies," Abizaid said. "They know what they are doing."
Wily and unpredictable, bin Laden is believed to be able to remain beyond the grasp of America's technology and might be burrowing into the ethnic Pashtun tribal areas located along the wild, 1,400-mile Afghan border. In the parlance of the U.S. military, the tribal areas are "ungoverned spaces."
Colonial Britain ceded the tribal areas to Pakistan a half-century ago, but Pakistani troops rarely venture there, and never without permission from the ethnic Pashtun leaders. The tribal areas are governed by centuries-old traditions. Disputes are often settled violently. Every Pashtun boy is taught to shoot. The men carry firearms openly.
Bin Laden spread his wealth and goodwill in the tribal areas during the 1980 jihad to oust the communist Soviets from Islamic Afghanistan. Those actions are still revered, as were more recent actions: At several places along the border where Arab fighters have been killed in firefights, local people have created informal shrines to the fallen al-Qaeda "martyrs."
The Pakistani government, wary of inflaming fundamentalist religious factions, has resisted even moderate U.S. troop deployments inside Pakistan. Only small teams of CIA, FBI and Special Forces personnel are now operating in the country, largely from an air base in Jacobabad.
The Pakistan border is a mere six miles from the U.S. base at Shkin. The tribal area there is called Waziristan, and it is practically a separate country. The international border is a meaningless line to Waziris and their allies, who cross the border at will.
Waziristan is a treacherous place. Al-Qaeda is openly recruiting across the border. Occasionally, the Waziri militias train their guns at the Americans. "During the day, they're Waziri border guards," said Lt. Chris Blaha, a soldier at Shkin. "At night, maybe they're soldiers-for-hire for al-Qaeda."
Although the tribal areas are hostile to intruders, the Pashtun code also calls for extravagant hospitality to friends and travelers. Many in Pakistan believe bin Laden is welcome in those areas.
"He could hide there easily," said Mohammed Sarwar Kakar, a Pashtun leader whose tribe lives on both sides of the border between Quetta, Pakistan; and Kandahar, Afghanistan. "Many tribal people are liking him."
Bin Laden is said to communicate to the outside world only with hand-carried notes, audio tapes and computer disks to avoid detection from the umbrella of U.S. electronic surveillance overhead.
"It's my opinion that he has cut himself off from much of his group," said Mullah Abdul Salam Rocketi, a former Taliban commander who now lives in Kabul. "Alone, he can hide himself. But with a group, he is more visible."
Some believe bin Laden is encamped in remote southeastern Afghanistan, especially Kunar province, with a force of 125 or more.
Others who know bin Laden and his tactics think it is more likely that he is traveling with a smaller group of perhaps two dozen people, including aides, bodyguards, couriers, a cook and a doctor who provides care for his ailing kidneys. Most of his family members - including four wives - are believed to be in Iran.
"I think he has a small force, lightly armed, a force that can move quickly and hide easily," said Rahimullah Yusufzai, a journalist based in Peshawar, Pakistan, who was the last person to interview bin Laden. He said bin Laden probably relocates only when a senior al-Qaeda leader is captured. "He knows the man will be aggressively interrogated, so that's when he moves."
Most theories hold that bin Laden is somewhere rugged and remote. But his most senior aides have been caught in Pakistan's biggest cities. Abu Zubaydah was nabbed in Faisalabad, and Ramzi bin al-Shibh was picked up in Karachi last year. Khalid Shaikh Mohammed, No. 3 in the al-Qaeda hierarchy, was discovered in a house in Rawalpindi in March.
Given all those key arrests in urban areas, some have suggested that bin Laden himself also might be living in a major city, hiding in plain sight.
"He could shave his beard, don a three-piece suit, and he could hide anywhere, probably forever," said Lt. Gen Hamid Gul, the former head of the Inter-Services Intelligence, the Pakistan spy agency that aided the Taliban's rise. "No one could detect him. He has been in the West and he knows the ways of the West."
Yusufzai believes bin Laden would not blend in very well even in a teeming city. Bin Laden is said to be about 6-foot-4, much taller than the average Pakistani or Afghan. Chronic back pain also forces the 46-year-old bin Laden to use a cane - another possible giveaway.
When bin Laden does move around the rugged border region, there are countless tracks and trails used by smugglers, secret paths that cannot easily be monitored.
"The border is very open, with Jeeps and trucks going back and forth," said Kakar, the tribal leader who is also a Pakistani senator from Baluchistan province.
Kakar thinks bin Laden is in Ribat, a desolate area where Pakistan, Afghanistan and Iran intersect. Also called the Triangle, Ribat is ruthlessly defended by heroin and opium smugglers.
It was in the Triangle, in the spring, that intelligence agents thought they had finally found bin Laden - "hot on his trail, just a few hours behind him," according to a Pakistani official close to the operation.
Using information gathered after the capture of Khalid Shaikh Mohammed in March, agents drew a bead on a donkey caravan moving in the Triangle. When a commando force swept down, however, the caravan was merely a gang of opium smugglers.
Few people who know the region believe the CIA's offer of a $25 million reward will have much effect. Kakar and others say any tribal people who give up bin Laden would be signing death warrants for themselves and their extended families. Gul, the retired general and spymaster, calls the reward "stupid."
"Osama can pay more than that to people hosting him," Gul said in an interview last week at his home in a military enclave in Rawalpindi. He first met bin Laden in 1992.
If U.S. agents or Pakistani troops should back bin Laden into a corner, diplomats in Islamabad say the Pakistanis will let the Americans mount the assault and take the credit for his death or capture. That tactic would give a huge political boost to President Bush while deflecting some of the heat President Pervez Musharraf's government is sure to get from religious hardliners and Islamist zealots.
Most believe that bin Laden will never be taken alive, that he has already given orders to his entourage to kill him first.
"This is a man resigned to his fate and not afraid to die for his cause," Yusufzai said at his home in Peshawar. "He knows his hideout will be found, but they've decided he won't be captured. He will always be seen as a martyr and an important symbol of resistance."
The United States has been pressing the Pakistanis to pursue terrorists more diligently in the tribal areas. Results have been slow in coming.
"We have a great level of cooperation with Pakistani forces," Abizaid said in an interview at Central Command's headquarters in Tampa, Fla. "But on the other hand, it has not actually reached the level of combined operations."
"The Pakistan army is now operating in tribal areas where it never operated before, where even the British didn't go," a Western diplomat said.
Pakistan's stepped-up efforts have raised anti-American protests.
"They are interfering with our freedom and sovereignty," said Maulana Fazl-ur Rehman, the cleric who heads the Jamiat-e-Ulama Islami, a fundamentalist party that claims several million supporters. "We won't act against them, but the people don't want them here, that's crystal clear."
Many believe that Musharraf's heart is not really into the hunt for terrorists, but he has been forced to cooperate with Washington to win billions of dollars in forgiven loans, trade concessions, military assistance, and fresh economic aid. A five-year, $3 billion aid agreement is in the offing.
"Musharraf is doing all this under duress," said Gul, no admirer of the president. "His mind is not with it."
While Pakistan moves slowly to extract some form of cooperation in its tribal areas, U.S. troops in Afghanistan are forbidden from crossing the border in pursuit of terrorists without the approval of the top command.
"The Pakistanis are trying to win hearts and minds, and we don't want to upset that diplomatic balance by going into the tribal areas," said Col. Barry Shapiro, the chief of staff of the coalition forces in Afghanistan.
Shapiro believes that U.S. forces are making inroads, building up trust among the Afghan public.
"I know the American people are expecting immediate results," he said. "But we're making incremental gains. We see more people coming in every day, offering up information."
Joe Galloway of the Inquirer Washington Bureau contributed to this article.