NSA documents seen by the Post said that the spooks carried out "full take," "bulk access" and "high volume" operations on Yahoo and Google networks.
Normally large-scale collection of Internet content would be illegal in the United States, but it seems that the operations took place overseas, where the NSA is allowed to presume that anyone using a foreign data link is a foreigner.
John Schindler, a former NSA chief analyst said the agency has platoons of lawyers, and their entire job is figuring out how to stay within the law and maximise collection by exploiting every loophole.
Google and Yahoo maintain fortresslike data centres across four continents and connect them with thousands of miles of fibre-optic cable.
Yahoo's internal network transmits entire e-mail archives from one data centre to another, which is when the NSA would pounce.
Tapping the Google and Yahoo clouds allows the NSA to intercept communications and read the material at its leisure. Spooks had to circumvent gold-standard security to get the data.
The weak point might have been some of the premium data links which the two search engines have been buying or leasing.
They had reason to think, insiders said, that their private, internal networks were safe from prying eyes, but apparently not.