From a Congressional Research Service Report for Congress published pre 9/11.
On reaching that watershed moment:
The National Security Agency (NSA), one of the largest components of the U.S. Intelligence Community, has reached a major watershed in its history. Responsible for obtaining intelligence from international communications, NSA’s efforts are being challenged by the multiplicity of new types of communications links, by the widespread availability of low-cost encryption systems, and by changes in the international environment in which dangerous security threats can come from small, but well organized, terrorist groups as well as hostile nation states.
On the scale of the problem: finding a needle in a haystack:
These links are not necessarily easy targets given the great expansion in international telephone service that has grown by approximately 18% annually since 1992. Intelligence agencies are faced with profound “needle- in-a-haystack” challenges; it being estimated that in 1997 there were some 82 billion minutes of telephone service worldwide.
On new technologies:
Fiber optics can carry far more circuits with greater clarity and through longer distances and provides the greater bandwidth necessary for transmitting the enormous quantities of data commonplace in the Internet age. Inevitably, fiber optic transmission present major challenges to electronic surveillance efforts as their contents cannot be readily intercepted, at least without direct access to the cables themselves.
On having to use communications data because they can’t break codes:
In some cases, NSA must resort to analyses of traffic patterns–who is communicating with whom, when, and how often–to provide information that may not be obtainable through breaking of codes and reading of plaintext.
On oversight and accountability
NSA and counterpart agencies in a number of other countries, especially Great Britain, have come under much criticism in the European Parliament for allegedly monitoring private communications of non-U.S. businessmen in a coordinated electronic surveillance effort known as Echelon in order to support domestic corporations. Some critics go further and charge that NSA’s activities represent a constant threat to civil liberties of foreigners and U.S persons as well. Though NSA has reassured congressional oversight committees that the Agency complies strictly with U.S. law, these controversies will undoubtedly continue.
You don’t need Snowden when you have the CRS.