One morning in June 2001, three months before the 9/11 attacks on the United States, I happened to be interviewing a senior official from the British Secret Intelligence Service, M.I.6. His current focus was the war on drugs, not international terrorism, but he shared a piece of information that united the two subjects.

A short time earlier, the official told me, the U.S. National Security Agency had intercepted a call between two satellite-telephone users in Afghanistan—the al-Qaeda leader Osama bin Laden and the Taliban leader Mullah Mohammed Omar. They had been discussing the Taliban’s ban on growing opium poppies, imposed the previous summer—a remarkably effective edict that had shrunk production in areas they controlled almost to zero.

According to the M.I.6 official, bin Laden sounded unhappy. “Why stop growing opium?” he asked. “Heroin only weakens our enemies.” There was no need to worry, Mullah Omar replied. The ban was merely a tactic. “There has been a glut, and the price is too low. Once the world price has risen, the farmers can start growing it again.”

The real lesson of this overheard conversation was not its specific content but the fact that it could be heard at all. Electronic eavesdropping clearly had potential in Taliban-controlled Afghanistan. But in the years before 9/11, when bin Laden’s terror plot was first being discussed, that potential remained limited. The reason was simple: Afghanistan had no cell phones, no Internet, and only a rudimentary landline network, which did not work at all outside the country’s largest cities. This could be remedied, however. Indeed, by the end of 1999, the Taliban government had embraced a full-fledged American scheme to install a modern cell-phone-and-Internet system in Afghanistan. It could have been up and running within months. The Taliban had already granted an exclusive license to a U.S.-owned firm, the Afghan Wireless Communications Company.

More to the point, electronic modifications concealed within the circuitry would have allowed every call and every e-mail emanating from Afghanistan to be relayed without interference to N.S.A. headquarters in Fort Meade, Maryland. “This project was a dream,” says one former senior F.B.I. counterterrorism specialist who knew about the scheme at the time. “To be able to wire up a country from ground level up—you don’t get too many opportunities like that.” No, you don’t. But at the critical moment, the Clinton administration put the project on hold, while rival U.S. agencies—the F.B.I., the N.S.A., and the C.I.A.—bickered over who should control it.

In the decade since 9/11, investigations by journalists and government commissions have explored the many missed opportunities to prevent bin Laden’s attacks. Overall, it is the story of a catastrophic failure to connect the dots. One can argue—and many have—that the connections emerge more visibly in retrospect than they ever did as events themselves unfolded. But the affair of the Afghan cell-phone network—put on hold until time ran out—falls into a category by itself. It was a course of action whose value and urgency were acknowledged by everyone, but it was impeded nonetheless. The cell-phone plan “was one tool we could have put in Afghanistan that could have made a difference,” a former C.I.A. official says. “Why didn’t we put it in? Because we couldn’t fucking agree.”

Until now, the existence of the cell-phone plan has remained a secret. It came close to surfacing during a court case in 2003, when the majority shareholder of Afghan Wireless, an Afghan-American named Ehsanollah Bayat, was sued in New York by his former British partners, Stuart Bentham and Lord Michael Cecil. But the Department of Justice persuaded a judge to invoke the State Secrets Privilege, a Draconian measure that allows the U.S. government to stop a case in its tracks on the grounds that allowing it to proceed would endanger national security. The entire legal record was sealed. Bentham and Cecil were issued gag orders, enforcing silence on pain of prosecution. It has therefore not been possible to speak to them about it. Bayat and his staff, for their part, failed to respond to numerous requests for comment. However, through interviews with other individuals who are not legally restricted, and through voluminous contemporary documents made available to me, it has been possible to assemble a narrative.