Art Keller had something to tell his girlfriend. It was early January 2006, and because of their busy work schedules, he decided to let her know via e-mail.

"I'm going to Pakistan for for a while for work," he wrote.

Keller was vague about what he would be doing there. He'd been dating this woman for a couple of months and had told her he worked for the State Department, in the Bureau of Non-Proliferation. He found that people's eyes glazed over when he told them this, and questions about his work dried up. As a CIA agent, this is precisely what he wanted.

"Dating is one of the challenges of the CIA," he says. "You're not supposed to tell people what you do right up to the point when you pop the question. At which point they wonder what else you've been lying about."

There was more that Keller, then 35, couldn't share with his girlfriend. Frustrated by the agency's bureaucracy, he planned to leave the CIA, and knowing that this was his final year, he had decided to take on one of the agency's most challenging assignments: to join the team of what most experts believe to be between 50 and 100 CIA agents and Special Operations Forces officers whose clandestine mission is to find and kill Osama bin Laden and other top Al Qaeda leaders in the hostile, barren border regions of Pakistan. In the CIA, it's always possible to find a post in Pakistan. Compared with, say, Paris or Manila or Moscow, the tribal regions of Pakistan are, in Keller's words, "not fun."

"The conditions are so miserable they'll take anyone," he says. "They're desperate for volunteers." Keller signed up for a 90-day tour—short by normal standards—and began preparing for his new job, which was to be the acting chief of one of the agency's bases in the heart of Al Qaeda and Taliban territory. On the other side of the border, in Afghanistan, U.S. and NATO forces operate openly. In Pakistan, American soldiers and agents must work in secret, because the government wants to avoid antagonizing Islamist militants.

By his own account, Keller was not the CIA agent with the best qualifications for this job. He spoke German and some Russian but none of the languages of the Middle East or South Asia. He was an expert on the proliferation of weapons of mass destruction, and like nearly every CIA agent in the post-9/11 era, he had intermittently worked on counterterrorism. He had worked for the CIA's Counterterrorist Center but had never officially been part of the group. Nor was he an expert on militant Islam, Al Qaeda, or Pakistan. Still, after four years as a CIA case officer, Keller was experienced enough to know that his mission would be daunting.

Keller had two months to prepare for his stint in Pakistan. He was working full-time as a CIA case officer in a midsize American city, meeting with agents he had recruited from local Middle Eastern communities as part of his WMD cases. He read as many books on Al Qaeda and Pakistan as he could, before leaving for Langley, Virginia, where he would spend his final week in the States reading the bin Laden files and researching local tribal dynamics at CIA headquarters.