U.S. authorities believe Iran probably unwound the CIA's asset network analytically -- meaning they deduced what Washington knew about Tehran's own operations, then identified Iranians who held that information, and eventually zeroed in on possible sources. This hunt for CIA sources eventually bore fruit -- including the identification of the covert communications system.

A 2011 Iranian television broadcast that touted the government's destruction of the CIA network said U.S. intelligence operatives had created websites for fake companies to recruit agents in Iran by promising them jobs, visas and education abroad. Iranians who initially thought they were responding to legitimate opportunities would end up meeting with CIA officers in places like Dubai or Istanbul for recruitment, according to the broadcast.

Though the Iranians didn't say precisely how they infiltrated the network, two former U.S. intelligence officials said that the Iranians cultivated a double agent who led them to the secret CIA communications system. This online system allowed CIA officers and their sources to communicate remotely in difficult operational environments like China and Iran, where in-person meetings are often dangerous.

A lack of proper vetting of sources may have led to the CIA inadvertently running a double agent, said one former senior official -- a consequence of the CIA's pressing need at the time to develop highly placed agents inside the Islamic Republic. After this betrayal, Israeli intelligence tipped off the CIA that Iran had likely identified some of its assets, said the same former official.

The losses could have stopped there. But U.S. officials believe Iranian intelligence was then able to compromise the covert communications system. At the CIA, there was "shock and awe" about the simplicity of the technique the Iranians used to successfully compromise the system, said one former official.

In fact, the Iranians used Google to identify the website the CIA was were using to communicate with agents. Because Google is continuously scraping the internet for information about all the world's websites, it can function as a tremendous investigative tool -- even for counter-espionage purposes. And Google's search functions allow users to employ advanced operators -- like "AND," "OR," and other, much more sophisticated ones -- that weed out and isolate websites and online data with extreme specificity.

According to the former intelligence official, once the Iranian double agent showed Iranian intelligence the website used to communicate with his or her CIA handlers, they began to scour the internet for websites with similar digital signifiers or components -- eventually hitting on the right string of advanced search terms to locate other secret CIA websites. From there, Iranian intelligence tracked who was visiting these sites, and from where, and began to unravel the wider CIA network.