People knowledgeable about Google and Yahoo’s infrastructure say they believe that government spies bypassed the big Internet companies and hit them at a weak spot — the fiber-optic cables that connect data centers around the world that are owned by companies like Verizon Communications, the BT Group, the Vodafone Group and Level 3 Communications. In particular, fingers have been pointed at Level 3, the world’s largest so-called Internet backbone provider, whose cables are used by Google and Yahoo.

The Internet companies’ data centers are locked down with full-time security and state-of-the-art surveillance, including heat sensors and iris scanners. But between the data centers — on Level 3’s fiber-optic cables that connected those massive computer farms — information was unencrypted and an easier target for government intercept efforts, according to three people with knowledge of Google’s and Yahoo’s systems who spoke on the condition of anonymity…

It is impossible to say for certain how the N.S.A. managed to get Google and Yahoo’s data without the companies’ knowledge. But both companies, in response to concerns over those vulnerabilities, recently said they were now encrypting data that runs on the cables between their data centers. Microsoft is considering a similar move.