It was January 2010, and investigators with the International Atomic Energy Agency had just completed an inspection at the uranium enrichment plant outside Natanz in central Iran, when they realized that something was off within the cascade rooms where thousands of centrifuges were enriching uranium.

Natanz technicians in white lab coats, gloves and blue booties were scurrying in and out of the “clean” cascade rooms, hauling out unwieldy centrifuges one by one, each sheathed in shiny silver cylindrical casings.

Any time workers at the plant decommissioned damaged or otherwise unusable centrifuges, they were required to line them up for IAEA inspection to verify that no radioactive material was being smuggled out in the devices before they were removed. The technicians had been doing so now for more than a month.

“We were not immune to the fact that there was a bigger geopolitical picture going on. We were definitely thinking … do I really want my name to be put on this?” – Eric Chien

Normally Iran replaced up to 10 percent of its centrifuges a year, due to material defects and other issues. With about 8,700 centrifuges installed at Natanz at the time, it would have been normal to decommission about 800 over the course of the year.

But when the IAEA later reviewed footage from surveillance cameras installed outside the cascade rooms to monitor Iran’s enrichment program, they were stunned as they counted the numbers. The workers had been replacing the units at an incredible rate — later estimates would indicate between 1,000 and 2,000 centrifuges were swapped out over a few months.

The question was, why?

Iran wasn’t required to disclose the reason for replacing the centrifuges and, officially, the inspectors had no right to ask. Their mandate was to monitor what happened to nuclear material at the plant, not keep track of equipment failures. But it was clear that something had damaged the centrifuges.

What the inspectors didn’t know was that the answer they were seeking was hidden all around them, buried in the disk space and memory of Natanz’s computers. Months earlier, in June 2009, someone had silently unleashed a sophisticated and destructive digital worm that had been slithering its way through computers in Iran with just one aim — to sabotage the country’s uranium enrichment program and prevent President Mahmoud Ahmadinejad from building a nuclear weapon.

But it would be nearly a year before the inspectors would learn of this. The answer would come only after dozens of computer security researchers around the world would spend months deconstructing what would come to be known as the most complex malware ever written — a piece of software that would ultimately make history as the world’s first real cyberweapon.