Last summer, an atomic bomb detonated in a city on the U.S. Eastern seaboard, killing tens of thousands and plunging the nation into despair. As first responders and the military grappled with the aftermath, elite teams of scientists raced to analyze the blast for clues to precisely what kind of bomb had gone off and who bore responsibility for the act.



That was the premise of an exercise—the first of its kind—held in July and August 2015 to test a new network of sensors that would collect data during a surprise nuclear strike. The Mighty Saber simulation was a sobering acknowledgment of many experts’ belief that an attack on U.S. soil is more likely than ever—yet tracing responsibility would be far harder than it was during the Cold War, when the chief threat was annihilation by the Soviet Union.



“The scenario has changed,” says Thomas Cartledge, a nuclear engineer with the U.S. Defense Threat Reduction Agency (DTRA) in Fort Belvoir, Virginia. “Now, if you see a mushroom cloud go off in New York City, you won’t know who did it, or what kind of weapon they used.”Possibilities include a warhead diverted from the U.S. arsenal or smuggled into the country by terrorists, or a bomb delivered by an enemy state such as North Korea, which has threatened to nuke the White House.



The conceivable need to unmask a perpetrator and mount a response is propelling the emerging area of postdetonation forensics. “Someone’s going to get the pointy end of the stick. You want to make sure the right entity gets it,” says Howard Hall, director of the Institute for Nuclear Security at the University of Tennessee, Knoxville. He and other nuclear detectives are devising new sensors, manufacturing artificial fallout to hone analytical techniques, and studying how the glass formed in the furnace of an atomic blast would vary depending on the nature of the bomb and the city where it detonated.



The most likely nuclear terrorism scenario, experts say, is a bomb set off on a city street. Past experience offers only a sketchy picture of the resulting devastation. The atomic bombs the United States dropped on Hiroshima and Nagasaki in 1945 detonated about 500 meters above those cities. During the subsequent half-century, while the United States refined its atomic arsenal, nearly all tests were in the air or underground, not in citylike environments. Researchers did study fallout and how it forms, but they were seeking clues about how to prevent or alleviate radiation illness, not identify the perpetrator. “Scientists were not interested in figuring out what kind of device had detonated, because they already knew that,” says analytical chemist Michael Kristo, a nuclear forensics expert at Lawrence Livermore National Laboratory in California.





Still, the testing program was a proving ground for postdetonation forensics. The U.S. national labs “put together some very good radiochemical procedures for analyzing debris,” says Hall, a radiochemist. Fallout is a mélange of the vaporized environment—soil and structures that were near the blast—laced with fission products (radioisotopes created when fissile materials like uranium or plutonium fission), activation products (radioisotopes formed when the blast radiation transmutes shielding and other bomb components), and residual nuclear material. The precise constituents vary according to a weapon’s design—whether it’s a simple gun-triggered uranium device, for example, or an intricate hydrogen bomb.



“Each type of weapon has a distinct fingerprint,” says Michael Pochet, a U.S. Air Force electrical engineer detailed to DTRA. In plutonium bombs, for example, the fissile isotope is plutonium-239, made in nuclear reactors and extracted by reprocessing spent fuel, which contains a mix of plutonium isotopes and other actinides like americium. Detecting those nuclei indicates that the bomb’s core was plutonium. Their proportions hold clues to the bomb’s history, says Joel Ullom, a physicist at the U.S. National Institute of Standards and Technology in Boulder, Colorado, who, with colleagues at Los Alamos National Laboratory in New Mexico, has developed a superconducting sensor that speedily differentiates plutonium isotopes.



The ratio between plutonium isotopes and americium-241, a decay product of plutonium-241, “can tell you the time since the plutonium was chemically purified,” Ullom says. Americium is removed during reprocessing, so as the freshly separated plutonium ages, americium starts accumulating again. Hall, meanwhile, is developing faster methods to analyze lanthanides, the 15 rare earth elements that, with the radioactive actinides, are key constituents of fallout. The mix of lanthanides and actinides reveals information about the weapon’s shielding, for example, and the energy of the neutrons that bombarded it. He intends to fit his gas phase separation apparatus onto a “flyaway lab”: a skid that can be deployed quickly in the event of an attack.



To ground-truth these analytical techniques, researchers at Livermore and other national labs are producing surrogate fallout representing different bomb types. The scientists have pressed into service the National Ignition Facility at Livermore, one of the world’s most powerful lasers, which Kristo calls “a ready source” of neutrons at energies comparable to those produced in the deuterium-tritium fusion reactions that power a hydrogen bomb.



Hall’s team is cooking up another type of test sample for postdetonation forensics: artificial melt glass. The real thing forms when an atomic inferno instantly melts anything having the misfortune of being at ground zero. The glass varies with the explosion site, but different bomb specs also produce unique melt glasses, providing clues about what happened. Hall’s group has developed a recipe book of melt glass for any geographic location based on a “witch’s brew” of the bomb’s fissile material and explosive yield, its detonation point, and the local geology and construction materials.



The team reproduced trinitite, the green-hued glass left by the Trinity test, the first U.S. nuclear detonation, which took place in 1945 at the White Sands Missile Range in New Mexico. They have also baked up specimens for Houston, Texas, where the glass-dominated architecture would yield a grayish glass if nuked, and for New York City, whose iron-heavy construction leads to a darker, volcanic-looking glass.



ATOMIC BLASTS ALSO UNLEASH an electromagnetic pulse—a blitzkrieg of gamma rays, x-rays, and radio waves that instantly fries most nearby electronics—as well as intense light, seismic waves, air pressure waves, and infrasound. All may provide information on the type of bomb and its origin. In the 1940s, scientists began designing sensors to capture these signals, first at White Sands and then primarily at the Nevada Test Site, where the United States detonated 928 bombs.



Now, DTRA is leading a government-wide effort to upgrade those sensors and link them up in an array, called Discreet Oculus, which can be deployed in and around cities. “We’ve repurposed the sensors for an urban environment,” Cartledge says. That required devising algorithms to account for how cityscapes deflect or absorb various types of waves, for instance, and filtering out noise from sources such as subways, the vibrations of which could interfere with interpreting vibrations from the detonation.



Mighty Saber set out to test the ability of Discreet Oculus to identify the type of bomb in a surprise attack. The exercise’s premise was that a bomb had been diverted from the U.S. arsenal and detonated. “We pulled in weapon designers to see what those signals would be,” Pochet says. In late 2013, several dozen experts began ginning up a fallout profile and modeling how waves would propa- gate and attenuate in a real U.S. city. DTRA won’t say which city it was; Cartledge refers to it as Gotham. “No city wants to know it was used as a model for a nuclear attack,” he says.



Based on these models, DTRA sent data simulating what Discreet Oculus sensors would record during the explosion to the Air Force Technical Applications Center on Patrick Air Force Base in Florida, which distributed it to four teams of experts from the center and the U.S. national labs. “We said, ‘Here’s the data, go and do your analyses’,” Cartledge says. The task was to identify the bomb, and time was of the essence. “In real life,” Pochet says, “we would be working against the clock, struggling to keep up with the news cycle.” The exercise ran for 25 days; all four teams figured it out, Cartledge says. He won’t specify how quickly but says, “We need to be faster.”



DTRA HAS ALREADY INSTALLED Discreet Oculus in several U.S. cities, where the arrays are undergoing testing. They are expected to be operational and transferred to the U.S. Air Force in 2018. DTRA has also begun working on a portable version called Minikin Echo that could be deployed for events like the Olympics.



Although postdetonation forensics may well finger a bomb design, that knowledge by itself wouldn’t always unmask the perp. A gun-triggered uranium bomb, for example, could be fashioned by any of a number of terrorist outfits with modest technological expertise, such as the Islamic State group, providing they can lay their hands on several kilograms of highly enriched uranium. That’s “where intel comes in,” Hall says. But to have any chance of unraveling the details of a nuclear attack, investigators have to lay the scientific groundwork—while hoping it will never be needed.