ASHINGTON, June 19 — The National Security Agency intercepted two cryptic communications on the day before the Sept. 11 attacks that referred to a major event scheduled for the next day, but analysts at the secret eavesdropping agency did not read the messages until Sept. 12, American intelligence officials said today.
One of the conversations intercepted by the agency on Sept. 10 said that "the big match" is scheduled for tomorrow, referring to Sept. 11. A second message called the next day "zero hour."
Agency analysts did not process, translate and review the intercepted Arabic communications until the day after the attacks, and so President Bush and other senior policy makers were not told of the possible warning until it was too late.
The conversations were from Afghanistan, and at least one suspected Al Qaeda operative has been tentatively identified as a participant, officials said. Osama bin Laden was not involved in the intercepted discussions, the officials added. The existence of the intercepted messages was first reported today on CNN.
Initially, American intelligence analysts could not identify any of the participants in the discussions, and are still uncertain about some of the people involved, officials said. American officials have also not proven conclusively that the conversations were referring to the Sept. 11 attacks on Washington and New York, but the evidence suggests that they were connected to the plot.
The delay in translating and processing the intercepted communications was normal given the priorities established within the agency at the time, officials said. Top priority intercepts can be translated and analyzed immediately, intelligence officials said — indicating that these intercepts did not receive the most urgent handling at the agency, perhaps because the agency had not identified the participants in the conversation at the time.
"On Sept. 12 when they looked at these intercepts, no one knew who these people were," said one official.
The disclosure of the intercepted communications came on the same day that Lt. Gen. Michael Hayden of the Air Force, the director of the security agency, and other top intelligence officials, testified before the joint Congressional committee investigating the government's performance before and after the attacks. Officials familiar with the Congressional hearings said that Mr. Hayden was questioned by lawmakers about his agency's intercepts during the morning hearing.
The agency now joins the Central Intelligence Agency and the Federal Bureau of Investigation in facing intense public scrutiny for apparently missing potential signals of a looming attack before Sept. 11. As Congress has geared up its investigation of the attacks over the past few weeks, the F.B.I. and C.I.A. have been heavily criticized for missteps in tracking Qaeda terrorists, sparking a major policy debate in Washington over revamping the intelligence community and improving coordination and information sharing inside the government.
Many critics of the intelligence community have predicted for months that it was only a matter of time before Congressional investigators would turn to the security agency and find evidence that that agency, specializing in eavesdropping and code breaking, had also failed to heed clues of a pending attack.
The critical problem facing the security agency is that it can collect far more data than is humanly possible to analyze and process in a timely manner. The agency is said to have more computing power than any other institution on earth, and, thanks to spy satellites and a wide array of other listening devices, it can vacuum the world's airwaves and computer lines without anyone noticing. It conducts intelligence collection on an industrial scale, intercepting millions of conversations over phone lines and the Internet each day.
But the real challenge is what to do with that raw information once it has been collected. With a budget that is several times larger than that of the C.I.A., the security agency is trying to use the latest in artificial intelligence and advanced software to identify which intercepted communications can be ignored and which require urgent attention. But even then, huge volumes of communications need to be translated from difficult languages like Arabic before they can be processed and sent to policy makers for review. Often, the nuances of conversations among suspected terrorists require the trained ear of senior analysts who have been listening to the same groups for years, and the sheer volume of material to cover again puts a strain on the N.S.A.
General Hayden has been candid about his agency's structural problems, and has been pushing to improve the agency and help it tap into the latest technology. In interviews, he has said that the agency's real problem is that during the cold war, its technology was far ahead of the rest of the world. But because of the high-tech revolution of the past 20 years, the commercial world has caught up — and in many respects surpassed — the agency. It now finds it difficult to keep up with the new communications technologies. "We used to be an information age agency in an industrial age, but now we are an information age agency in the information age," General Hayden once observed.
United States intelligence officials said that even if the two conversations were analyzed and sent to policy-makers on Sept. 10, the information was so vague that it would not have been enough to point to the attacks on the World Trade Center and the Pentagon.
"Even if Mike Hayden had run into the president with these, what could they have done?" asked one official.
In another development, law enforcement officials said today that the F.B.I. was planning to launch a large monitoring effort during the Fourth of July holiday and will monitor and watch parades and festivals to guard against possible terrorist attacks.
The officials said that the high-intensity counterterrorism operation was a precautionary step that was not based on a specific threat but on interviews with detainees at Guantanamo Bay in Cuba and past intelligence reports indicating that Qaeda followers regard the holiday as a significant period for a possible attack.
F.B.I. officials have asked each of the bureau's 56 field offices to submit plans for monitoring events in their regions.