For years, an enduring mystery has surrounded the Stuxnet virus attack that targeted Iran’s nuclear program: How did the U.S. and Israel get their malware onto computer systems at the highly secured uranium-enrichment plant?

The first-of-its-kind virus, designed to sabotage Iran’s nuclear program, effectively launched the era of digital warfare and was unleashed some time in 2007, after Iran began installing its first batch of centrifuges at a controversial enrichment plant near the village of Natanz.

The courier behind that intrusion, whose existence and role has not been previously reported, was an inside mole recruited by Dutch intelligence agents at the behest of the CIA and the Israeli intelligence agency, the Mossad, according to sources who spoke with Yahoo News.