Why haven't we found.

More than three years after the September 11 attacks turned Osama bin Laden into the most wanted man in the world, the search for him remains stalled. The manhunt is frustrated by the remote topography of his likely Pakistani sanctuary, stymied by an Al Qaeda network that remains well-financed and disciplined, sidetracked by the distractions of the Iraq war, and, perhaps most significant, limited by the deep suspicion of the United States among Pakistanis.

Meanwhile, from wherever he is hiding, bin Laden continues to release tapes encouraging Muslim militants to attack America. Last fall, he warned the U.S. to change course to prevent future attacks. In late December, he issued a message threatening January's elections in Iraq.

Pakistan began last spring to search for bin Laden and flush out the Al Qaeda forces that had escaped from Afghanistan. The Pakistani Army declared victory in November and announced that bin Laden was not in Pakistan. But American intelligence officials are confident that he is--and that he is as dangerous as ever.

MORE 'SPECTACULAR' ATTACKS?

The war in Afghanistan, which began in November 2001, inflicted severe damage on Al Qaeda, intelligence specialists believe. But they say bin Laden, from his presumed hiding place on the Pakistani side of the Afghan-Pakistan border, still controls an elite terrorist cell devoted to attacking in the U.S. Officials believe he hopes to use the group for another "spectacular" event, like the 9/11 hijacking plot. American analysts say the U.S. has failed to penetrate this special Al Qaeda unit and has no idea when or where it will try to strike.

In late 2003, the CIA established a series of small secret bases in the mountains of northwest Pakistan. The agency believed bin Laden was being sheltered there by local tribesmen and foreign militants. But since the bases opened, the CIA officers there have been strictly supervised by Pakistani officials. As a result, officials say it has been virtually impossible for the Americans to gather intelligence effectively.

The place suspected of being bin Laden's hideout, in the shadow of the Hindu Kush mountain range, is in one of the most isolated corners of the world. Pakistan's frontier is a barren terrain of mountains and mud. The fiercely independent ethnic Pashtun who live there are farmers and smugglers, most of them poor and illiterate. Local religious leaders preach a radical Islamic ideology that portrays the U.S. as determined to destroy Muslim culture.

The tribal areas in the region have been a virtual no-man's-land for American forces, making them a natural haven for Al Qaeda figures who fled Afghanistan after the battle of Tora Bora in 2001.

Pakistan does not permit American military and intelligence forces in Afghanistan to cross the border to go after militants. This makes it relatively easy for Taliban and Al Qaeda fighters to attack American bases in Afghanistan, and then escape to the safety of Pakistan.

Since spring 2002, northwest Pakistan has been "the hub of Al Qaeda operations in the whole world," says one senior Pakistani official. The foreign militants are flush with cash, travel in small groups at night, use a sophisticated code when communicating, and have access to laptop computers, Pakistani military officials say. The network has even sent e-mail messages, letters, and DVDs to Pakistani soldiers fighting in the tribal areas urging them not to kill their fellow Muslims on behalf of America.

'POTENT FORCE'

In allowing the CIA bases in Pakistan, President Pervez Musharraf is trying to balance his alliance with the U.S. with his need to avoid setting off a broader insurgency in the border region, where the central government is resented for its long neglect.

American intelligence officials have been skeptical of the Pakistani effort, though. To rout Al Qaeda and find bin Laden is going to require a much more sustained campaign, say officials from both countries. Pakistani officials and others say economic development, locally elected government, and full integration of the tribal areas into Pakistan are the only way to eradicate militancy from the isolated area.

"To really neutralize and eliminate them [Al Qaeda], it will have to be a lot more effort," says Talat Masood, a Pakistani political analyst and former general. "They are still a very potent force."

James Risen and David Rohde of The New York Times reported from Washington and Peshawar, Pakistan, respectively, for this article.