In November 2008, Abid Naseer, a Pakistani student living in Manchester, England, began to e-mail a Yahoo account that was ultimately traced back to his home country. The young man’s e-mails appeared to be about young women — Nadia, Huma, Gulnaz and Fozia — and which of them would make a “faithful and loving wife.”

British investigators later determined that the four names were code for different types of explosives and that a final April 2009 e-mail announcing a “marriage to Nadia” between the 15th and the 20th was a signal that a terrorist attack in England was imminent, according to British court documents.

It is unclear exactly how British intelligence services linked the Pakistani e-mail address, [email protected], to a senior al-Qaeda operative who communicated in a kind of pigeon code to his distant allies. But the intelligence helped stop the plot in England, and the address somehow made its way to the National Security Agency at Fort Meade, Md.

A few months later, the NSA was monitoring the Yahoo user in Pakistan when a peculiar message arrived from a man named Najibullah Zazi, an Afghan American living in Aurora, Colo. He asked about “mixing of [flavor and ghee oil] and I do not know the amount, plz right away.”