On Christmas Eve 2003, Gen. Michael Hayden, the director of the secretive U.S. National Security Agency, made a secure phone call to his British counterpart, David Pepper, the director of the Government Communications Headquarters.

“Happy Christmas, David,” Hayden said, speaking to Pepper from NSA headquarters at Ft. Meade, Maryland, about 20 miles from the Capitol in Washington, D.C. Such social calls weren’t unusual. The NSA and GCHQ were the closest of allies in a global hunt for the phone calls, emails, and other electronic communications of spies and terrorists.

But Hayden had more on his mind than season’s greetings. In recent days, the NSA had been collecting what Hayden would later describe as a “massive amount of chatter”—phone calls and emails from terrorists—that suggested al Qaeda was planning multiple attacks inside the United States, timed to the holidays.