Compound 19 was not an easy place to enter.
For a start, it was located within a city entirely closed off to the outside world. The complex was not on any maps. Even if you could get close enough to see the military facility, high walls, guard dogs and armed soldiers would ensure you got no further.
If only what was contained within those walls had as much trouble getting out as others had to get in, up to 100 people might not have died.
This weekend marked 38 years since the Sverdlovsk leak. Also known as the "biological Chernobyl" the incident at a top secret Soviet weapons factory was covered up for more than a decade.
When people started dying in the surrounding area, authorities claimed they had eaten a batch of tainted meat. The reality was far more sinister.
"I walked up to one of the patients. I can still see him before my eyes, he is still alive," Dr. Marguerita Ilyenko, who worked at the local hospital, told Canada's CBC of those days in early April 1979.
"But right there in front of me, I can already see death spots forming all over his body. On his neck, on his back, doctors know what this means. Then he began to vomit and died. It was a very quick death."
Spring had sprung in the southern Russian city, then part of the Soviet Union, located where the European and Asian continents meet.
Now called Yekaterinburg, Sverdlovsk is - and was - a bustling city, the fourth largest in Russia.
Unbeknown to the residents of Sverdlovsk, as they flung open their windows to take advantage of the warmth, a deadly cloud of anthrax spores was wafting its way towards them.
It wasn't the only accident to happen at germ factories - people had died at similar facilities in the west too - but the scale of what happened was unprecedented then or since.
"All those who fell ill were people who, for one reason or another, had been outside at night or early in the morning," Viktor Romanenko, the then deputy chief of the region's epidemiological service, told the BBC in 2001.
The detail of exactly what occurred behind those walls is scant. All records were destroyed. But what has been gleaned is that a simple error led to a multitude of deaths.
Military Complex 19 was busy concocting ever more virulent strains of anthrax, a highly infectious bacteria that literally eats away at the internal organs. While naturally occurring anthrax can kill one-in-four people, the easily inhalable weaponised form has a far higher fatality rate.
The Soviets aimed to retro fit some intercontinental ballistic missiles, replacing the nuclear warhead with a cylinder of anthrax spores ready to be dropped on western cities.
While much of the population would perish, the cities themselves would survive.
"The primary reason the anthrax bacterium is dangerous is that it makes a powerful toxin. It also makes a very resistant form of the bacterium that is called a spore," wrote Professor Julian Rood, Professor of Microbiology in the Department of Microbiology at Monash University.
"The spore form is able to survive adverse environmental conditions for decades. The spores can enter the soil and then, many years later, can infect another animal and make that animal sick."
What exactly happened on April 2, 1979, is disputed. Some said there had been an explosion at Compound 19. But according to Ken Alibek, a former head of the Soviet Union's bio weapons program who later moved to the US, just a large filter shielded the laboratory where the anthrax spores were produced and the outside world.
On that day, wen the machines were routinely turned off, a technician removed one of the filters. This vital information did not reach the staff on the next shift and when the machines were turned back on, the missing filter wasn't noticed.
The deadly spores silently made their way from their top secret lab, past the guards and over the barbed wire fence.
Alexandra Chizhova worked in the city's ceramics factory.
"She was a hearty, 50-year-old woman when she arrived home from her shift, feeling terribly unwell," wrote Michael Halon in the Sunday Times in 2001.
"She had a high temperature, severe cramps and was having difficulty breathing. Two days later she was dead and her death certificate claimed she'd had pneumonia." She was, in fact, one of the first victims.
Six people would die in Compound 19 itself and a further 11 in an adjoining military facility. At least 68 people would perish as the anthrax spores attacked them from the inside out. It's possible the death toll reached more than 100. Scores of farm animals also succumbed.
"Nobody told us what was going on, we just saw people; in masks washing our houses. There was no information," Yelena Klyuchagina, a local resident told the BBC.
Officially the citizens had all been struck down by poisoned black market meat. The deaths were listed as simple "infections" by the local bureaucracy. No mention was made of anthrax.
Given that, in 1972, the USSR has signed a biological weapons control treaty, the Government was in no hurry to confess.
"We never believed it," Nikolai Burmistrov, who also worked at the ceramic plant, told the independent. "Ordinary people catch on fast. We knew lots of people who had eaten the meat did not get anthrax. After a few days, we guessed it must have come from Compound 19."
It was only when scientists examined where the victims had been on that day that a pattern became clear.
At the time all the dead had been in a stretch of land stretching from the compound, past homes and the ceramics factory, to open countryside. In that narrow channel, death was carried gently on the warm breeze.
The bodies were buried together, isolated in a distant corner of the city's cemetery in coffins lined with lime. Local people were mass inoculated.
As the Cold War began to thaw a local Communist Party chief in Sverdlovsk rose to power.
Boris Yeltsin would later become the first President of post-Soviet Russia. In 1992, he admitted that tainted meat was not to blame for the deaths. That the most likely explanation was indeed that a cloud of deadly spores, meant to be emptied onto teeming American cities, has instead been emptied on Russia's own backyard.
If the wind was blowing in the opposite direction, towards the city centre, the spores would strike down thousands.
Today, in modern day Yekaterinburg, Compound 19 still exists. Behind high walls and steel gates its secrets are still hidden away.