| I still say it was the Ruritanians... |
In brief, the case for the United States having designed and developed Stuxnet is as follows: First, neither the United States nor Israel wants Iran to develop nuclear weapons. The worm, then, is seen as likely part of a covert strategy to delay or destroy Iran's nuclear infrastructure while stopping short of war.
The weapon was designed to target a specific version of the Siemens SCADA programmable logic controls (PLC) operating a specific configuration and number of cascading centrifuges found in Iran. Some analysts point to the fact that there were vulnerability assessments being run at Idaho National Labs on Siemens PLC software. Others note that the design of the cyber weapon closely fit Richard Clark's description in Cyber War of a well-designed and ethically thought out weapon limiting collateral damage due to a vast army of lawyers scrutinizing the effects. The malware-analyst community, meanwhile, points to digital code strings such as "b:myrtus" taken from biblical events important to Israeli identity. And, as the story goes, after the political decisions, vulnerability assessments, and weapon design took place, either an Iranian agent was found to take the USB memory stick into the nuclear facility, or all the computers around the plants were infected with Stuxnet via the conficker worm.
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