The agency was capturing a copy of all the data passing over the telecommunications links and then filtering it in AT&T facilities that housed systems that were able to filter data packets at high speed.

Documents taken by Edward J. Snowden and reported by The Washington Post indicate that, seven years after Mr. Klein first described the N.S.A.'s surveillance technologies, they have been refined and modernized.

“From Echelon to Total Information Awareness to Prism, all these programs have gone under different names, but in essence do the same thing,” said Chip Pitts, a law lecturer at Stanford University School of Law.

Image The chief executive of Verizon, Lowell C. McAdam. Credit... Julie Jacobson/Associated Press

Based in the Denver suburbs, Level 3 is not a household name like Verizon or AT&T, but in terms of its ability to carry traffic, it is bigger than the other two carriers combined. Its networking equipment is found in 200 data centers in the United States, more than 100 centers in Europe and 14 in Latin America.

Level 3 did not directly respond to an inquiry about whether it had given the N.S.A., or the agency’s foreign intelligence partners, access to Google and Yahoo’s data. In a statement, Level 3 said: “It is our policy and our practice to comply with laws in every country where we operate, and to provide government agencies access to customer data only when we are compelled to do so by the laws in the country where the data is located.”

Also, in a financial filing, Level 3 noted that, “We are party to an agreement with the U.S. Departments of Homeland Security, Justice and Defense addressing the U.S. government’s national security and law enforcement concerns. This agreement imposes significant requirements on us related to information storage and management; traffic management; physical, logical and network security arrangements; personnel screening and training; and other matters.”