WASHINGTON — A secretive U.S. eavesdropping agency monitored telephone conversations before Sept. 11 between the suspected commander of the World Trade Center and Pentagon attacks and the alleged chief hijacker, but did not share the information with other intelligence agencies, U.S. officials said Thursday.

The officials, speaking on condition of anonymity, said the conversations between Khalid Shaikh Mohammed and Mohammed Atta were intercepted by the National Security Agency, or NSA, an intelligence agency that monitors and decodes foreign communications.

The NSA failed to share the intercepts with the CIA or other U.S. intelligence agencies, the officials told Knight Ridder. It also failed to promptly translate some intercepted Arabic language conversations, a senior intelligence official said.

The officials declined to disclose the nature of the discussions between Mohammed, a known leader of Osama bin Laden's al-Qaida network who is on the FBI's Most Wanted Terrorists list, and Atta, who piloted one of the planes that hit the World Trade Center. Khalid Shaikh Mohammed is believed to be hiding in Pakistan.

Another intelligence official, speaking on condition of anonymity, said it was "simply not true" that the NSA monitored the conversations and failed to share the information with other intelligence agencies.

An NSA spokesperson said that as a rule "we neither confirm or deny actual or alleged intelligence operations." She declined to say more.

The disclosure of the intercepts marks the first time the NSA has been dragged into the controversy over whether U.S. intelligence and law enforcement agencies may have had information prior to Sept. 11 that could have helped avert history's most deadly terrorist attacks.

The CIA and FBI are already under fire for mishandling information that could have provided clues to the attack or to the presence in the United States of some of the hijackers.

A U.S. official familiar with the NSA intercepts said they might have contributed to the building of a "threat matrix" — or overall picture — if added to the CIA and FBI information.

The House and Senate intelligence committees are scrutinizing the NSA, CIA and FBI as they examine what the government knew or should have known about the terrorist threat prior to the attacks.

Committee investigators are aware of the intercepts of the conversations between Mohammed and Atta and the NSA's failure to share them with other intelligence agencies, said the senior U.S. intelligence official.

The official said investigators had determined that some intercepts were not translated in a timely fashion. In other cases, he said, NSA analysts apparently did not recognize the significance of what they had.

The congressional investigators' initial conclusion is that the NSA's human and technical systems are not up to the job of translating, sorting, analyzing and disseminating the ever-increasing avalanche of data the agency collects, the official said.

"The basic task of an intelligence analyst is to take a pile of stuff from different sources and look for a pattern," said the senior intelligence official. "In order for that to work, the analysts need to see everything that's available. But with the system we have, they almost never do, because the system doesn't work right."

The NSA is the U.S. intelligence community's premier code-breaker and foreign communications eavesdropper. It also protects American communications systems.

Run by the Department of Defense, the NSA is the largest of the 13 agencies that make up the U.S. "intelligence community." It was created in 1952 and was once so secret that it was jokingly referred to as "No Such Agency." Although its budget and staff size are secrets, it is believed to have more than 30,000 employees and an annual budget of more than $5 billion.

From its headquarters in Fort Meade, Md., outside Washington, the agency operates a globe-spanning network of satellites and ground stations that monitor millions of telephone and fax calls, radio signals and e-mails. It also processes signals collected by ships, submarines and aircraft.

Acres of underground super computers that recognize keywords such as names search through the data to find communications such as conversations between terrorists.

The NSA is prohibited by law from monitoring calls to and from the United States without special court orders.

The high-tech spy agency is credited with coups that helped win the Cold War, such as listening to Soviet leaders' talking on the telephone in their limousines. Another NSA victory was catching Libya red-handed in the 1986 bombing of a Berlin discotheque.

More recently, the NSA alerted the CIA, FBI and other agencies to a meeting in Kuala Lumpur, Malaysia, in January 2000 between a bin Laden lieutenant and two of the Sept. 11 hijackers.

But in recent years, the agency has struggled under post-Cold War budget and staff cuts. Meanwhile, a global explosion in telecommunications technologies, from digital telephones to fiber optic networks, has made the NSA's task more difficult.

Other NSA problems include aging equipment and a lack of translators.

"They're still going through these . . . changes, the extremely difficult changes, converting from the Cold War to the war on terrorism," said James Bamford, author of two books on the NSA.

In fighting terrorism, the agency focused on watching for large transfers of cash and explosives, Bamford said. But the Sept. 11 plot involved neither. It revolved around small terrorist cells whose members rarely talked and whose methods were no more sophisticated than "getting together to knock over a gas station," he said.

The senior intelligence official said that when the NSA monitored their conversations, Mohammed was overseas and Atta was in the United States.

Mohammed was included on the FBI's Most Wanted Terrorist List when it was published in October because he had been indicted on charges of being involved in a failed 1995 plot to bomb 11 U.S. airliners flying over the Pacific Ocean on a single day. The U.S. Justice Department has offered a $25 million reward for him.

Based on information developed since Sept. 11, including recent interrogations of a senior al-Qaida member captured in Pakistan, U.S. officials have concluded that Mohammed had overall command of the Pentagon and World Trade Center attacks.

The FBI says Mohammed is from Kuwait, but the Kuwaiti government denies he is a citizen.

According to U.S. officials, Atta, an Egyptian, was the leader of the 19 hijackers who crashed two airliners into the World Trade Center, another into the Pentagon and a fourth into a field in Pennsylvania.

U.S. investigators believe Mohammed might have met several times in Germany in 1999 with Atta or members of Atta's al-Qaida cell.