About two weeks after it struck Neda, a control engineer who worked for the company popped up on a Siemens user forum on July 22 complaining about a problem that workers at his company were having with their machines. The engineer, who posted a note under the user name Behrooz, indicated that all PCs at his company were having an identical problem with a Siemens Step 7 .DLL file that kept producing an error message. He suspected the problem was a virus that spread via flash drives.

When he used a DVD or CD to transfer files from an infected system to a clean one, everything was fine, he wrote. But when he used a flash drive to transfer files, the new PC started having the same problems the other machine had. A USB flash drive, of course, was Stuxnet’s primary method of spreading. Although Behrooz and his colleagues scanned for viruses, they found no malware on their machines. There was no sign in the discussion thread that they ever resolved the problem at the time.

It’s not clear how long it took Stuxnet to reach its target after infecting machines at Neda and the other companies, but between June and August the number of centrifuges enriching uranium gas at Natanz began to drop. Whether this was the result solely of the new version of Stuxnet or the lingering effects of the previous version is unknown. But by August that year, only 4,592 centrifuges were enriching at the plant, a decrease of 328 centrifuges since June. By November, that number had dropped even further to 3,936, a difference of 984 in five months. What’s more, although new machines were still being installed, none of them were being fed gas.