In October, 2005, a radiation sensor at the Port of Colombo, in Sri Lanka, signalled that the contents of an outbound shipping container included radioactive material. The port’s surveillance system, installed with funds from the National Nuclear Security Administration, an agency within the Department of Energy, wasn’t yet in place, so the container was loaded and sent to sea before it could be identified. After American and Sri Lankan inspectors hurriedly checked camera images at the port, they concluded that the suspect crate might be on any one of five ships—two of which were steaming toward New York.

It’s thought that there are some fifty-four thousand licensed batches of radioactive materials in the U.S. that could be used in a dirty bomb. JOHN RITTER

Sri Lanka is a locus of guerrilla war and arms smuggling. It is not far from Pakistan, which possesses nuclear arms, is a haven for Al Qaeda, and has a poor record of nuclear security. The radiation-emitting container presented at least the theoretical danger of a “pariah ship,” Vayl Oxford, the director of the Domestic Nuclear Detection Office, which is part of the Department of Homeland Security, said. It seemed plausible, if unlikely, that Al Qaeda or rogue Pakistani generals might load a bomb onto a cargo vessel. Within days, American satellites located the five suspect ships and intelligence analysts scrutinized their manifests; a team at the National Security Council took charge. One ship, it learned, was bound for Canada, and another for Hamburg, Germany. The White House decided to call in its atomic-bomb squad, known as NEST, the Nuclear Emergency Support Team—scientists who are trained to search for nuclear weapons. One team flew to Canada and a second to Europe, where it intercepted one of the ships at sea before it could reach Hamburg. They found nothing.

The United States Coast Guard stopped the two New York-bound ships in territorial waters, about ten miles offshore; from that distance, if there was a nuclear weapon on board a detonation would cause relatively little harm. Scientists boarded the vessels, shouldering diagnostic equipment, but these ships, too, turned out to be clean; as it happened, the offending vessel was on an Asian route, and its cargo was scrap metal mixed with radioactive materials that had been dumped improperly. The entire episode, which was not disclosed to the public, lasted about two weeks.

This sometimes nerve-racking exercise resulted in no more than the disposal of some radioactive waste. It was also the first major defensive maneuver triggered by a shield that the United States is attempting to build as a defense against a clandestine nuclear attack. The idea, in essence, is to envelop the country in rings of radiation detectors and connect these sensors to military and police command centers, which would then respond to unexplained movements of nuclear material. The project, comparable in ambition to ballistic-missile defense, is the first of its kind in the atomic age. The plan has already attracted criticism from some scientists and defense strategists, primarily because, as with missile defense, the project promises to be expensive and would require leaps of ingenuity to overcome technical problems presented by the laws of physics.

Still, with little public discussion this “layered defense,” as it is described by its proponents, is being deployed. The federal government has distributed more than fifteen hundred radiation detectors to overseas ports and border crossings, as well as to America’s northern and southern borders, domestic seaports, Coast Guard ships, airports, railways, mail facilities, and even some highway truck stops. More detectors are being distributed each month. NEST and the Federal Bureau of Investigation maintain a permanent team to respond to events in Washington and along the Northeast Corridor; a second team trained to dismantle nuclear weapons is based in Albuquerque, and eight other teams able to diagnose radioactive materials operate on continuous alert elsewhere in the country. Since the terrorist attacks of September 11, 2001, NEST teams have been deployed about twice a year because of specific threats reported by intelligence agencies, including at least two instances, apart from the Sri Lankan episode, where they boarded a ship approaching the United States. NEST units also discreetly screen vehicles, buildings, and people at designated events such as political conventions and the recent N.B.A. All-Star Game, in Las Vegas. In the United States alone, the sensors generate more than a thousand radiation alarms on an average day, all of which must be investigated.

The world, it turns out, is awash in uncontrolled radioactive materials. Most are harmless, but a few are dangerous, and many detectors are still too crude to distinguish among different types of radiation; they ring just as loudly if they locate nuclear-bomb material or contaminated steel or, for that matter, bananas, which emit radiation from the isotope potassium-40. So far, the result has been a cacophony of false alarms, which, in most cases, are caused by naturally occurring radiation that has found its way from soil or rock into manufactured products such as ceramic tiles. In addition, people who have recently received medical treatments with radioactive isotopes such as thorium can set off the detectors. At baseball’s All-Star Game in Detroit in 2005, unobserved NEST scientists screened tens of thousands of fans entering the stadium, and their sensors rang just once—reacting to the former Secretary of Energy Spencer Abraham, who was radioactive from a recent doctor’s visit.

Detritus from nuclear commerce that has slipped through American and international regulatory systems is another periodic source of alarms, and one that has proved to be a greater cause of concern. Virtually none of the loose material detected so far would be useful to a terrorist seeking to build a fission weapon—a bomb of the sort that was dropped on Hiroshima. A disquieting fraction of it, however, might be useful for what the American defense bureaucracy calls a “radioactive dispersal device,” more commonly known as a dirty bomb. There is recent evidence, too, that Al Qaeda-inspired radicals are pursuing such a weapon.

The term “dirty bomb” can refer to a wide variety of devices, but generally it describes one that would use a conventional explosive such as dynamite to release radioactive material into the air. The initial explosion and its subsequent plume might kill or sicken a dozen or perhaps as many as a few hundred people, depending on such factors as wind and the bomb-maker’s skill. If the weapon was particularly well made, employing one of the most potent and long-lived types of radioactive materials that are used in medicine and in the food industry, it might also cause considerable economic damage—perhaps rendering a number of city blocks uninhabitable. Radioactive ground contamination cannot easily be scrubbed away, so it might be necessary to tear down scores of buildings and cart the rubble to disposal sites. It’s easy to imagine what the impact of such an attack would be if the contaminated area was, say, a quarter of the East Village, or the Seventh Arrondissement of Paris.

Charles Ferguson is a former nuclear submarine officer trained in physics; he left the Navy for a career in security studies and is currently a senior fellow at the Council on Foreign Relations. In 2003, he co-wrote an unclassified report titled “Commercial Radioactive Sources: Surveying the Security Risks.” About two years later, F.B.I. agents working on an international terrorism case asked to meet with him. They brought a document showing that some of his report had been downloaded onto the computer of a British citizen named Dhiren Barot, a Hindu who had converted to Islam. Barot, it turned out, had been communicating with Al Qaeda about a plan to detonate a dirty bomb in Britain, and he had used a highlighting pen on a printout of Ferguson’s study while conducting his research.

The report described how large amounts of certain commercial radioactive materials might pose a danger to a terrorist who tried to handle them. “This seems to have worried him,” Ferguson told me, referring to Barot, “so he decided to look at smoke detectors.” Some detectors contain slivers of americium-241; the isotope’s constant emission of radiation creates a chemical process that screens for smoke. Barot informed his Al Qaeda handlers that he was thinking about buying ten thousand smoke detectors to make his bomb. In fact, to make a device that would be even remotely effective, Ferguson said, he would have had to buy more than a million. “Either his reading comprehension was poor or he was evading the assignment,” Ferguson told me. In Britain, last October, Barot pleaded guilty to terrorism-related charges.

Barot appears to have been only marginally more competent than Jose Padilla, the hapless American convert to Islam who travelled to Pakistan, met with Al Qaeda leaders, and then flew to the United States, where he was arrested amid great fanfare, in June 2002. John Ashcroft, then the Attorney General, held a press conference in which he accused Padilla of “exploring a plan” to build a dirty bomb, charges that were later omitted from an indictment against him.

The Barot and Padilla cases raise a strategic question—whether it is worth setting up an expensive, imperfect system whose effectiveness would be greatest against slow-witted terrorists. The Bush Administration is now spending about four hundred million dollars annually on radiation-detector research, but nuclear physicists who have studied the technology disagree about how discriminating these sensors might become. One point on which everyone agrees, however, is that, of all the potentially dangerous radioactive isotopes, it will always be most difficult to detect highly enriched uranium-235, one of the two materials, along with plutonium, used to make fission weapons. Unless it is being compressed to explode, highly enriched uranium is a low-energy isotope that does not emit much radioactivity—it is “dull,” in the lexicon employed by scientists in the field. This makes it relatively easy to shield inside lead casing, or to mask by surrounding it with brighter isotopes. Plutonium, by comparison, is fairly bright, and many of the most dangerous isotopes that could be used in dirty bombs, such as cesium 137 and cobalt 60, are brighter still. Radiation sensors, then, will always be more effective against a Dhiren Barot than against, say, the Pakistani nuclear scientist Abdul Qadeer Khan, a metallurgist who has spent many years studying fission weapons and highly enriched uranium, as well as the challenges of international smuggling.

It is common, in defense studies, to evaluate an adversary on the basis of capability and intent. Pakistan has a nuclear-weapons capability, but its government, however fragile it may be, is presumed to have no hostile intentions toward the United States. Al Qaeda, on the other hand, has demonstrated hostile intentions but has little known nuclear capability. Osama bin Laden has declared that the acquisition of nuclear weapons is a religious duty, and it is well documented that he tried to buy uranium during the mid-nineteen-nineties while he was living in Sudan. (Like many other would-be purchasers of black-market nuclear material, he apparently fell victim to a scam.) After September 11th, bin Laden met with Pakistani nuclear scientists to discuss weapons issues. More recently, Al Qaeda-inspired radicals have sought nuclear materials. “We know they have a significant appetite and they have been searching for different materials, in different venues, for the past several years,” Vahid Majidi, an assistant director of the F.B.I., who is in charge of the bureau’s newly formed weapons-of-mass-destruction directorate, told me. “The question becomes our vigilance and their ability to execute.”

Last September, the Nuclear Threat Initiative posted a translation of a message that appeared on the Web and was attributed to Abu Ayyub al-Masri, the leader of Al Qaeda in Iraq. The speaker called for experts in “chemistry, physics, electronics, media and all other sciences, especially nuclear scientists and explosives experts.” He continued, “We are in dire need of you.… The field of jihad can satisfy your scientific ambitions, and the large American bases are good places to test your unconventional weapons, whether biological or dirty, as they call them.”

The available evidence, then, suggests that while jihadi leaders might like to acquire a proper fission weapon, their pragmatic plans seem to run to dirty bombs—a more plausible ambition. Among other things, the international nuclear black market holds more promise for dirty-bomb builders than for those who are interested in fission weapons. In all the cases of nuclear smuggling reported to the International Atomic Energy Agency since the collapse of the Soviet Union, none have involved significant amounts of fissionable materials. (There have been at least two cases in which a seller possessing small amounts of highly enriched uranium promised that he could get much more but was arrested before the claim could be tested; the most recent of these occurred in the former Soviet republic of Georgia, in 2006.) By comparison, the I.A.E.A. has recorded about three dozen black-market smuggling incidents through 2004 involving radiological isotopes in quantities that would be useful for a destructive dirty bomb, according to European diplomats who have analyzed the records. It would not be simple to build a damaging device with these materials. Still, Peter Zimmerman, who served as the chief scientist of the Senate Foreign Relations Committee from 2001 to 2003, said, “I think there are Al Qaeda people who, given finely divided material, could think of very creative and malicious ways to use it. Why hasn’t it happened? The answer is we’ve been lucky.”

The Bush Administration has not assigned the same urgency to the dirty-bomb threat that it has to the threat of a terrorist attack using a fission weapon. Fred Iklé, who served as the Under-Secretary of Defense for Policy in the Reagan Administration and has consulted on homeland-defense matters for the Bush Administration, told me that he and his colleagues have been considerably more concerned about a full-blown nuclear-weapons conspiracy, which would have the potential to trigger a worldwide economic depression and force millions of Americans to flee major cities. By contrast, even the worst dirty-bomb event, Iklé said, would be less than “a Katrina.”

Last year, analysts at the Department of Homeland Security divided the threat of a weapon-of-mass-destruction attack against the United States into two categories, “catastrophic” and “limited,” according to Maureen I. McCarthy, a senior adviser in the department’s intelligence and analysis office. A catastrophic attack, in this taxonomy, would cause ten thousand or more casualties and fifty billion to a hundred billion dollars in economic damage, and would produce a “major global policy shift,” McCarthy said last November, at an intelligence symposium. A limited attack might produce a hundred to a thousand casualties and would be confined to a single region, although it might also have “global political consequences.” The D.H.S. intelligence analysts placed a fission-weapon attack, the use of some biological agents, and an outbreak of hoof-and-mouth disease in the catastrophic category (the latter in part because it might require the closure of national borders for up to ninety days). Dirty bombs fell into the limited category. From the very beginning, fear of a fission bomb and its consequences has influenced American thinking about the costs and benefits of possible defenses against nuclear terrorism.

The Washington office of Los Alamos National Laboratory is in a modern building on the south side of the Mall, near a busy hotel. Richard Wagner has a spacious office on the second floor, which he has filled with color photographs of nature scenes. He is seventy years old, a trim man with a white mustache and a calm, precise demeanor. Wagner is a physicist who entered the field of nuclear weapons during the nineteen-sixties. He rose to become the deputy director of Lawrence Livermore National Laboratory and, for five years during the Reagan Administration, served as the Pentagon’s principal civilian adviser on nuclear weapons. He chaired an intelligence advisory board at the Pentagon during the Clinton years. At that time, he undertook the first of three studies on how the United States might erect a defense against a nuclear sneak attack. As much as anyone, Wagner is convinced of the need to employ radiation sensors in a national shield.

Wagner recalled, when I visited him on a recent wintry afternoon, that his interest in nuclear terrorism began during the early nineteen-seventies, when an F.B.I. agent arrived at Livermore carrying an extortion note. The F.B.I. man wanted to know if the threat, which involved a plan to blow up a nuclear device, was plausible. It was not, as it happened, but the incident, and several others like it during that period, got Wagner and a colleague at Livermore, Bill Nelson, thinking about what they would have done if they ever faced a serious case.

The subject had received remarkably little attention. In 1946, Robert Oppenheimer, the physicist who supervised the building of the first atom bombs, told Congress that three or four men “could destroy New York” by sneaking a nuclear weapon into the city. When a senator asked how such a weapon, smuggled in a crate or a suitcase, could be detected, Oppenheimer replied, “With a screwdriver.” It was not until the early seventies that the issue was revived inside the defense bureaucracy—stimulated, in part, by the publication of John McPhee’s “The Curve of Binding Energy,” which drew on interviews with the theoretical physicist Theodore B. Taylor, an innovator in nuclear-weapons design. Taylor spoke about the possibility that an individual, perhaps an American citizen, could build a fission bomb. In one striking passage, he holds a sliver of metallic uranium-235 in his hands as he speculates, “If ten per cent of this were fissioned, it would be enough to knock down the World Trade Center.” As a result of these warnings, Wagner recalled, “the government was getting more sensitive to the possibility that this might happen.”

At the time, the dominant fear was that a bomb-builder would issue an extortion demand; the government would then have to find him in a hurry and dismantle his weapon. “Our job was to search, and then, if we ever found anything, do something safe with it,” Wagner said. “It was the threat object that was fixed, and we were moving. And the idea of it being the other way around, the threat object moving toward the U.S. or around the U.S., and the detectors being fixed, which is part of the current paradigm—I don’t remember that as being much in our thinking.” To address such possibilities, Wagner helped to create NEST.