I believe this was already explained, but I can't remember where. Siemens had been working with the Idaho National Laboratory to identify vulnerabilities in the

I believe this was already explained, but I can't remember where. Siemens had been working with the Idaho National Laboratory to identify vulnerabilities in the PCS systems, which were then used in the exploits that Stuxnet leveraged. The US intelligence agencies that developed Stuxnet could have used INL as a front for interacting with Siemens or they could have simply taken the vulnerabilities after they had been identified as part of that joint effort. Either way I'm sure there was help from Siemens somewhere throughout the process, but not necessarily for the explicit purposes of creating the Stuxnet worm.