Searching for digital fingerprints; also see Flame: Israel rejects link to malware cyber-attack (BBC)
"Because Stuxnet and Duqu were written on the same platform and share many of the same fingerprints in their source code, researchers believe both were developed by the same group of programmers. Those developers have never been identified, but researchers have cited intriguing bits of digital evidence that point to a joint American-Israeli effort to undermine Iran’s efforts to build a nuclear bomb.Researchers Link Flame Virus to Stuxnet and Duqu - NYTimes.com
For example, researchers at Kaspersky Lab tracked the working hours of Duqu’s operators and found they coincided with Jerusalem local time. They also noted that Duqu’s programmers were not active between sundown on Fridays and sundown on Saturdays, a time that coincides with the Sabbath when observant Jews typically refrain from secular work."