In early April of 1979, a mysterious plague wafted through the Soviet city of Sverdlovsk, now known as Ekaterinburg. At least 66 people and an unknown number of animals were struck with a vague illness and then swiftly died. Soviet officials blamed tainted meat sold on the black market. But a 1992 investigation by a Harvard researcher finally aired the real killer: a plume of anthrax spores accidentally released from a clandestine bioweapons plant in town, known as Compound 19.

It was one of the largest inhalation anthrax outbreaks in history—and one of the few traces of the Soviet’s secret dabbling in bioweapons research—all thanks to botched air filter maintenance. The slow-moving death cloud that seeped from the compromised ventilation system left a trail of carnage 30 miles downwind of Compound 19. If the breeze had blown directly toward the center of town, which sits about 230 miles north of Russia’s border with Kazakhstan, thousands could have been killed.

Despite the devastation, many details about the accident remain murky—along with what else might lurk within the Ministry of Defenses’ Compound 19. Such facilities and research should have been shut down in 1975, after the international Biological Weapons Convention. But the Sverdlovsk accident and other hushed accounts suggest that covert Soviet biowarfare research continued in the following decades. In fact, reports from Soviet defectors in the 1990s tell of a vast, highly funded program involving tens of thousands of researchers working on biological weapons to lob at America and other targets. Russian officials have refused outside access to lingering facilities, and there’s no guarantee that research has completely stopped.

With the threat still looming 37 years after the Sverdlovsk accident, scientists finally have an unhindered view of the deadly germ inadvertently unleashed: The full genome sequence of the anthrax strain has been decoded and published in the microbiology journal mBio. The blueprints of the bioweapon will make it easy for scientists to identify the deadly bug and its relatives if they’re ever released again. And the state of the germ’s genetic code, i.e., the extent of human tampering, offers clues to the Soviet strategy.

It’s no mystery why the Soviets would mess with anthrax to begin with, though. Anthrax infections, caused by the bacteria Bacillus anthracis, have long been tempting candidates for biowarfare and bioterrorism. Usually, infections are extremely rare and are typically spread from infected animals through undercooked meat, hides, or wool. But the bacteria have the useful ability to form nefarious spores. These nearly indestructible pods can lie dormant in soil—or perhaps warheads—for decades. But once inhaled, ingested, or introduced to an open wound, the spores come alive, producing deadly toxins and infection.

In the crosshairs

Antibiotics can squash an anthrax infection. There’s a vaccine as well. In fact, it was the Soviet’s tinkering with a vaccine strain of B. anthracis that provided the first hints of biowarfare research. In the 1960s, researchers purposefully made a vaccine strain resistant to multiple antibiotics. The stated goal was to make a vaccine that could survive the antibiotics used to treat an active anthrax infection, which would not be resistant to drugs. But the researchers went further. They genetically tweaked the drug-resistant bacteria to evade the immune responses trained by a vaccine. The explanation was that researchers wanted to study how B. anthracis alters the immune system. However, the final product was infectious bacteria that could evade immune responses as well as multiple drugs.

And B. anthracis wasn’t the only germ the Soviets were rumored to have tweaked like this. In their extensively researched 2012 book The Soviet Biological Weapons Program: A History, scientists and security experts Milton Leitenberg and Raymond Zilinskas uncovered reports of plague bacteria (Yersinia pestis) that could go undetected by standard diagnostic tests. The Soviets also genetically engineered the bacteria Francisella tularensis, which causes tularemia, to be resistant to multiple drugs. And it’s likely they weaponized Legionella bacteria, which causes Legionnaire’s disease, to evade immune responses while pumping out deadly toxins. The result was a killer germ that produced little in the way of symptoms—the biological equivalent of a gun with a silencer.

Such reports had researchers deeply worried about what monstrous manipulations were in the Sverdlovsk strain. Samples of the deadly B. anthracis survived in formaldehyde-fixed tissues from some of the victims, collected by Russian investigators in the 1990s. But the aged and chemical-soaked samples were tricky to analyze, and some of the germs’ DNA was damaged. Previously, researchers could only pick out snippets of the bacteria’s genome sequence—enough to prove that the bacteria was there.

But in the new study, researchers led by geneticist Paul Keim of Northern Arizona University were able to use the latest, far more sensitive sequencing technology to pull out a full genome’s worth of DNA code. The code wasn’t perfect; there were still damaged spots from the formaldehyde fixing. But it was an otherwise complete sequence.

Nesting it in the tree of other anthrax genome sequences, the researchers found it was a relative of the established vaccine strain that the Soviets had—a very close relative. To the researchers’ surprise, they could find no evidence that the Soviets had manipulated the genetics of the germ. In fact, there were just a handful of small genetic differences between the Sverdlovsk strain and its closest relative. It was as if the strain was in an evolutionary standstill.

This could suggest that what was released was a master batch of naturally nasty anthrax spores. It’s well known that the more anthrax is grown in lab, the more it adapts to its cushy petri dish lifestyle and becomes less deadly. “All of this is highly suggestive of a weapons program that identified a suitable strain, maintained master cell stocks to avoid extensive passage, and per- formed minimal manipulations in order to maintain virulence,” Keim and his colleagues concluded. “This strategy must have been used to produce large quantities of highly virulent material, as evidenced by the anthrax deaths in 1979.”

The finding also hints that the research may have hit snags. For instance, security analysts assumed that the Soviets would at least try to make an anthrax strain resistant to antibiotics. However, like lab growth, getting those genetic changes can make the bacteria less lethal—one step forward, one step back. But if the master strain was what leaked out, there’s no telling what small-batch mutants might still be locked up in Compound 19.

In their book, Leitenberg and Zilinskas concluded that the current lack of transparency raises concern. “One must assume that whatever genetically engineered bacterial and viral forms were created… remain stored in the culture collections of the Russian Federation Ministry of Defense.”

mBio, 2016. DOI: 10.1128/mBio.01501-16 (About DOIs).

Editor’s note: This story has been updated to clarify the language of biowarfare.