Just how extensive is NSA's spy program?

A week after a domestic-spying scheme by the National Security Agency was disclosed, the details remain shrouded in secrecy.

President Bush forcefully defended the operation in a press conference on Monday, but offered few clues about how it worked in practice. Neither did Attorney General Alberto Gonzales, except to say that "it is probably the most classified program that exists in the United States government."

But some technologists and civil libertarians, using clues that dribbled out in press briefings and news articles, are concluding that the operation involves widespread monitoring of millions of e-mail messages and telephone conversations that cross any U.S. border.

Some officials have hinted that the eavesdropping has spanned technology far more advanced than telephones.

"The clues are piling up that vacuum-cleaner style dragnets are what's at issue," John Gilmore, co-founder of the Electronic Frontier Foundation, said in a mailing list message on Thursday. "Perhaps they've pointed the NSA vacuum cleaner straight into all U.S.-based international telecommunications."

The longstanding purpose of the NSA is what intelligence agencies call "signals intelligence." In practice, that means vacuuming up data that can be gleaned through eavesdropping on microwave links, satellite signals, and fiber and copper underwater cables. In the past, NSA officials have assured jittery politicians and the public that its massive, supersensitive electronic ear is not aimed at U.S. citizens.

Now, however, questions are bubbling up about whether Bush's secret order authorized the NSA to aim that ear at Americans when at least one party to the conversation was in another country.

Gilmore suspects that the NSA may have assembled a database of every phone call that enters and leaves the United States, coupled with similar automated storage of at least the "header" information of e-mail messages. (Header information includes the To:, From: and Subject: lines.)

As evidence for that belief, he points to a paragraph in a New York Times article on Tuesday. It cites unnamed officials as saying the NSA system can identify "hot numbers"--the telephone numbers of suspects. Although the article doesn't go any further, Gilmore suspects the same spotting scheme applies to e-mail message as well.

It's happened before. The NSA's Project Shamrock, for instance, involved opening all telegrams that entered the U.S. or traversed the country. Reforms initiated by the Senate's Church Committee in the 1970s were an attempt to curb the NSA's increasing domestic surveillance.

For its part, the Bush administration and its allies deny the charges. "A massive indiscriminate net including thousands of domestic or U.S.-to-U.S. connections is wrong," Rep. Peter Hoekstra, a Michigan Republican who heads the House Intelligence Committee, told reporters Wednesday.

"It deals only with international calls," said Michael Hayden, former NSA director and current principal deputy director for national intelligence. "It is generally for far shorter periods of time. And it is not designed to collect reams of intelligence, but to detect and warn and prevent about attacks."

Some officials have hinted that the eavesdropping has spanned technology far more advanced than telephones. In defending the administration's decision not to seek court orders for spying, Gonzales referred to the influence of "changing technology" and the "new kind of threat" that groups like al-Qaida pose.

So does a 2003 letter (click for PDF) to Vice President Dick Cheney, written by Sen. Jay Rockefeller, a Democrat from West Virginia, and made public this week. In it, Rockefeller wrote that he's not "a technician" but is worried about "the direction the administration is moving with regard to security, technology, and surveillance."

A quick political outcry
The situation has incited fury among some members of Congress, with Democrats particularly vocal, and may have been responsible for derailing full reauthorization of the Patriot Act before 16 of its sections expire on Dec. 31.

In the U.S. House of Representatives, a group of Democratic leaders urged House Speaker Dennis Hastert in a letter to hold hearings and to appoint an independent panel to investigate the matter. Sen. Arlen Specter, a Pennsylvania Republican, has already announced plans for hearings.

House Democratic Leader Nancy Pelosi of California has also asked NSA to declassify a letter she sent to its intelligence director--along with the administration's response--in hopes of shedding public light on her concerns.

A large part of the dispute centers around why Bush did not abide by the Foreign Intelligence Surveillance Act of 1978 (FISA), which requires the government to obtain a court order before carrying out electronic surveillance. In order to receive the warrant, prosecutors must go to a secret court and demonstrate probable cause that the party it wants to monitor is a "foreign power" or an "agent of a foreign power."

The law sets the bar higher for spying on U.S. citizens, largely because of the Fourth Amendment, which reiterated that U.S. persons--including citizens and certain other permanent residents--cannot be subject to unreasonable searches and seizures.

The Bush Administration has admitted that it did not seek warrants from the FISA court for this surveillance program, but says that it's not legally bound to do so. "The president has the inherent authority under the Constitution, as commander-in-chief, to engage in this kind of activity," Gonzales said this week.

But from a technical perspective, FISA was written in a way that permits only individuals to be targeted. It doesn't envision a dragnet. So the questions, and speculation, will continue.

CNET News.com's Anne Broache contributed to this report.