Shhhh! There's 'no such agency'

The National Security Agency deciphers coded messages from America's adversaries.

(CNN) -- It's America's biggest spy organization and the principal translator of messages sent between and from foes of the United States.

Now, with the release of information that the National Security Agency received foreshadowing terror messages on September 10, people are asking questions about the third leg of the country's intelligence tripod, which includes the CIA and the FBI.

Created in 1952, the National Security Agency is twice the size of the CIA and possibly more secretive. So clandestine is the NSA's operations that people in the intelligence loop even joke that NSA really stands for "no such agency." It will not disclose its precise size, number of employees or budget.

So what goes on at the "nonexistent" agency? What's its purpose? And what problems does it face?

The NSA serves as the country's cryptologic organization, deciphering and writing messages in secret code. Its twofold mission includes designing coded U.S. information and defense systems while protecting them from code-breakers and trying to break the codes of other countries.

The NSA is the largest employer of premier mathematicians and code breakers in the country, its Web site claims.

On September 10, the NSA heard two chilling messages during its routine eavesdropping. The conversations were in Arabic, but officials are not sure who was talking.

One of the intercepts said, "The match begins tomorrow." The other said, "Tomorrow is zero hour." The intercepts however, were not translated and analyzed until September 12 -- one day after the attacks.

A lawmaker said that revelation emerged during a House-Senate intelligence committee meeting. NSA Director Michael Hayden said the 48-hour turnaround is a result of having a large volume of incoming information.

The NSA used to principally spy on the Soviets, but that is no longer the case. The United States has dozens of enemies, and the NSA is watching all of them.

Besides volume, technology complicates the NSA's mission. The worldwide move to digital, rather than analog phones, makes eavesdropping tougher. Fax machines and fiber optic cables are a much harder to tap. And the Internet creates mountains of public -- not secret -- data that needs to be analyzed