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.

“One more thing, David,” Hayden said after the two men exchanged pleasantries. “We actually feel a bit under threat here. And so I’ve told my liaison to your office that should there be catastrophic loss at Ft. Meade, we are turning the functioning of the American [signals intelligence] system over to GCHQ.”

There was a long pause as Pepper absorbed what his American colleague had just told him.

The word “catastrophic” suggested some event that would destroy the NSA’s headquarters, which housed the computer equipment that made sense of all that chatter streaming in through U.S. sensors, listening posts, and computer implants. The only reason to turn over that signals intelligence system—something that had never been done—would be if the NSA was no longer capable of performing its mission.

“Mike,” Pepper inquired, “Do you guys know something we don’t?”

Hayden thought they did.

Amid that massive amount of chatter, NSA officials and others were seeing the same threat made repeatedly: that al Qaeda terrorists were planning to detonate a nuclear device in or around Washington, D.C.

The intelligence wasn’t specific enough to guide officials to a bomb. But the threat was also too significant to ignore.

“Intelligence said there was very likely” a nuclear device that “could affect command and control systems in Washington,” a former U.S. official who was directly involved in efforts to verify the intelligence and prepare for a possible detonation told The Daily Beast. “The concern was that it would be catastrophic.”

The 2003 nuclear threat, which has received almost no public attention in the years since, was described to The Daily Beast by three former U.S. officials, two of whom were privy to contemporaneous intelligence on the matter and were working frantically on efforts to determine whether there really was a nuclear device hidden in the United States or on its way here. Four other former officials also described intelligence reporting in the same period that alerted officials to a different threat against commercial airliners. Collectively, all said this was the period of highest anxiety since the 9/11 attacks, a moment when some in the U.S. government were all but convinced that al Qaeda was poised to strike at any moment.

The intelligence about a possible nuclear attack was taken seriously enough that teams of inspectors from the Energy Department fanned out in several major U.S. cities, including Washington, Los Angeles, and New York carrying radiation detection equipment concealed in luggage.

“I didn’t even go home that Christmas,” said a second former official who was monitoring reports from the detection teams. “There was all kinds of intelligence, targeting different cities.”

The inspectors “were trying to figure out, is there some credibility to this information,” the first former official said. “It was taken as very significant, because if it was true we’d have a lot of bases to cover.”

At Ft. Meade, that meant preparing for the worst. Losing the ability to analyze global communications was “unthinkable” for Hayden, the former official said. Then, and now, signals intelligence are the primary source of information about terrorists and their plots. If Ft. Meade were destroyed, the entire U.S. security apparatus would be crippled.

“David, just a precaution,” Hayden told his counterpart. “But if we go down, you run the show.”

***

Hayden described his fears of a “catastrophic” attack and his plan to hand over direction of signals intelligence to the British in his recent memoir, Playing to the Edge, and he also recalled the story in greater detail at a book talk in March. But his remarkable account appears to have gone unnoticed by the press and most of the public.

In his book and public remarks, Hayden never mentioned intelligence that suggested a nuclear device might be in play. In an interview with The Daily Beast, he said “I just don’t remember” what the specific nature of the threat was, though he vividly recalled every other detail of his unprecedented planning and conversations with Pepper.

Hayden did allow, however, that he was worried that a large explosion could be set off on Maryland Route 32, the state highway that runs “right by some key buildings” at Ft. Meade, and effectively take the agency offline.

But it wasn’t just NSA computers that Hayden was worried about. He emphasized that in asking Pepper to take over, he intended GCHQ to assume “direction” of U.S. signals intelligence—that is, the job of the director, Hayden, who in the event of a massive explosion might be dead.

The first former official confirmed that as part of Hayden’s worst-case-scenario planning, he presumed he would no longer be around to give orders.

For officials trying to make sense of the threat, the lack of specificity contained in the chatter was frustrating. And the volume of information set people on edge.

Multiple “streams” of intelligence all seemed to point to some plot, though no one could say for sure how it would happen. Was the device already in the country? Was it going to be shipped in through a port or over a land border? And was the device a traditional nuclear weapon or a “dirty-bomb,” a conventional explosive that would disperse radioactive material and potentially render whole city blocks uninhabitable?

“As we looked at the reports, we thought, ‘Wow, if we read this right, this could come at us a lot of ways,’” the first former official said.

Experts were divided over whether al Qaeda really had the mix of talent and resources to obtain a nuclear device. A third former official said that in the months preceding the holiday scare, lawmakers and intelligence officials had debated the matter in closed-door sessions, and that many considered it an urgent question.

“It was very clear there was a lot of anxiety around nukes or dirty nukes, and there were interagency debates about al Qaeda’s capabilities,” the former official said. “Some pooh-poohed the idea that anyone but a government could obtain a weapon. Others said, ‘No, no, it’s possible.’”

Among those who thought this way, even the slightest shred of credible intelligence was taken seriously, the former official said. And within that camp, another debate emerged. Would terrorists smuggle the device into the United States? Or was it possible that al Qaeda had already deployed or recruited agents in the country to build the bomb here?

Against that backdrop of uncertain possibilities, threat intelligence started streaming in around mid-December, mainly from intercepted phone calls and emails, that seemed to fit with al Qaeda’s nuclear ambitions.

“We knew that al Qaeda was trying hard to obtain a weapon,” the first former official said. “There was a plausible theory, and there was evidence to support that theory.” Given the stakes, and the risk of losing a major portion of U.S. spying, Hayden had little choice but to prepare for the worst.

***

Hayden’s plan made sense. The GCHQ is the closest technological analog to the NSA. “It’s not like this was something they couldn’t handle,” Hayden told The Daily Beast. The British and the Americans not only share information, but they each possess a formidable, global-intelligence gathering system.

There was also no one else to take the baton in the United States. At the time of the holiday threat, the NSA was building an alternate facility, a kind of mini-backup of the site at Ft. Meade that was far away enough from Washington to avoid an attack, but also not so far that it couldn’t be reached by key personnel within a few hours. (Former officials declined to say where the facility was located.)

The facility was designed to handle about 80 percent of NSA’s mission but in a much smaller physical space, Hayden said. But as of December 2003, it still wasn’t fully up and running.

“We were also pushing a lot of missions out to the RSOCs,” Hayden said, referring to the NSA’s regional operations centers, in Texas, Hawaii, and Georgia. “But that wasn’t complete, either.”

That left Ft. Meade as the only U.S. location that could fully perform the critical signals intelligence mission. “It was closer to a single point of failure than we wanted it to be,” Hayden said. “Since we hadn’t done all our dispersal [to the alternate sites] and were still vulnerable, the next best thing was to contact the British.”

The public knew nothing about nuclear inspection teams or doomsday prepping at Ft. Meade, but even from the outside, the world appeared more dangerous than usual.

At the White House, the CIA, and the barely-year-old Homeland Security Department, top officials and their analysts were busy tracking another stream of intelligence that suggested imminent attacks against airliners. Security concerns led to at least 15 flight cancellations on aircraft bound from France, the United Kingdom, and Mexico, stranding anxious holiday travelers.

On Dec. 22, Homeland Security Secretary Tom Ridge had announced the national terror alert was being raised to its second-highest level, owing to a “substantial increase” in reports pointing to “near-term attacks that could either rival or exceed what we experienced on September 11.” Ridge told the public that security was being enhanced at airports as well as “around other transportation systems and infrastructure,” and that border patrol agents and the Coast Guard were being dispatched to monitor seaports and land border crossings.

Was Homeland Security also worried that al Qaeda might smuggle a device into the United States? The focus on land borders was notable, at least in hindsight, in light of a claim eight months later from a captured al Qaeda operative named Sharif al-Masri, who told his CIA interrogators that al Qaeda had intended to “transport a nuclear device or material… to the United States, possibly via Mexico,” according to a declassified Justice Department memo. The claim also seemed to fit with the intelligence community’s broader assessment that al Qaeda was trying to obtain a nuke.

At the White House, reporters pressed for more details about suspected terror plots. Americans were told to go about their plans, but as always, remain vigilant.

Despite these public warnings, and the obvious fears about airplane attacks, the suspected nuclear threat was never revealed. It also appears to have been known only by a few people even within senior ranks of the U.S. government. Three former high-level officials who worked on the airliner threat, including one at the White House, said they had no recollection of a nuclear weapons scare or of Hayden’s plans with GCHQ.

They all remembered the airliner threat, and specifically that al Qaeda would try to crash planes in the U.S., as it had on 9/11, or blow them up mid-air. The CIA developed a particularly novel theory that al Qaeda was telling its operatives which international flights to attack by sending coded messages in the “crawl” at the bottom of Al Jazeera’s television news broadcasts.

“Hayden actually seemed somewhat skeptical of this threat,” a former White House official told The Daily Beast. “These [NSA] guys knows something about steganography,” the practice of concealing messages within text that isn’t secret. “The technical guys were all sitting around the table kind of rolling their eyes when the CIA guys were talking about it.”

“That would be a true story,” Hayden told The Daily Beast. “We just didn’t see it. There was no evidence that would cause us to” think it was accurate.

***

But then, what was the evidence that led some to believe that the nuclear threat might be real? The experts at the NSA may have been able to discount coded messages on television, but no one could say with certainty that there wasn’t a nuclear device hiding somewhere in Washington. Ultimately, the intelligence was startling and maddeningly vague; as it often was, and is. In the end, the worst fears turned out to have been based on intelligence that wasn’t credible, the former official said.

Those Energy Department teams who fanned out across the country in search of an al Qaeda nuke found nothing but a shiny, cylindrical radium pellet, used to treat uterine cancer, that had been picked up years earlier by a homeless man and stashed in a rented storage locker in Las Vegas. (The Washington Post, which reported the discovery, was also the first and only publication to note that the inspectors had been dispatched during the holiday scare. But the newspaper’s account, which seems to have gone almost entirely unnoticed, said that the inspectors were sent because of “the belief among officials” that al Qaeda was eager to obtain a nuclear device. In fact, the intelligence suggested the terror group might already have one, the former officials told The Daily Beast.)

No one knew at the time that the leads would go nowhere or that the frightening reporting would turn out to be baseless, or maybe just bluster by al Qaeda operatives trying to sow the seeds of confusion. Still, the 2003 holiday scare stands out as one of the most dangerous periods since the 9/11 attacks, even if the threat was merely perceived and not real.

Many of the people who spent their holidays wondering when the attack might come hadn’t talked about the story in years. The plane threat and the nuclear scare had faded into the annals of myriad plots that never materialized. The chatter, once deafening, dropped to a murmur and was soon drowned out by a new chorus when the Homeland Security Department warned just a few months later that signs once again pointed to an attack.

Had officials come forward and shared their worst fears with the American public, it’s not clear what good would have come of it. No one in government could say for sure where the device might be located. Imagined fears of a devastating nuclear attack would be quickly superseded by the real threat of mass panic.

But security fatigue was also on the rise. By late 2003, a palpable skepticism greeted the Bush administration’s warnings about orange terror alerts and “near-term” attacks that could rival 9/11. The next summer, the terror alert was again raised to orange before the Democratic National Convention, and the president’s opponents accused him of conjuring up vague threats to scare voters into re-electing him, rather than taking a chance on his Democratic rival. Even Ridge, the homeland security secretary, would later write in his memoir that he was pressured to ratchet up the alert level on the eve of the 2004 election.

No doubt Hayden and others have factored all this and more into their own thinking. When warning about a vague, mysterious threat can make you seem paranoid or mendacious, the best bet is often to keep quiet. That’s probably why many stories of near-misses and false alarms in the past 15 years of a war on terrorism have gone unreported.

Hayden chose not to talk about the 2003 threat until he published a book, and his critics will surely detect some opportunism there. They have excoriated him for his leadership of the NSA’s StellarWind program, which secretly monitored communications of millions of Americans, and have said he tried to whitewash the CIA’s torture of terrorism suspects after he became the director of that agency.

But occasionally, behind the chatter that we’ve come to anticipate—and often ignore—someone decides to take it seriously. On Christmas Eve 2003, that was Hayden, who said he opted to follow the simple maxim, better safe than sorry.

“In retrospect, we were probably being too cautious and too alarmist,” Hayden said of his plan to hand off signals intelligence to GCHQ. But the 9/11 attacks two years earlier had permanently altered his perspective. On the evening of Sept. 11, 2001, after sending all non-essential agency employees home, Hayden ordered that black-out curtains be tacked up in NSA’s glass windows. That’s standard procedure for protecting buildings during an air raid. As the sun went down that day, Hayden thought his building might still be a target. Two years later, he still did.

“9/11 was a surprise,” Hayden said. And he was done with surprises.