Known as Able Danger, the project was created by the U.S. Army in 1999 and used to compile primarily publicly available information in a computer program and later map out a network of people with ties to known terrorists. But sometime between December 2000 and March 2001, the project was disbanded--for unclear reasons--and up to 2.5 terabytes worth of records, both electronic and hard copy, were ordered destroyed by the Army.

Sen. Arlen Specter, the Pennsylvania Republican who chairs the Senate Judiciary Committee, convened a hearing Wednesday to ask whether Sept. 11 ringleader Mohammed Atta's photo was, in fact, part of that data, and, above all, why the data was not shared with law enforcement agencies but deleted instead.

But, in an 11th-hour decision that surprised and perplexed senators, the Defense Department prohibited five key witnesses from testifying at the hearing, citing concerns about classified information, Specter said.

Some of the witnesses were nonetheless present in the audience. At one point, Specter even asked them to stand and be identified. But the senators could question only people who said they could speak for those witnesses, including Mark Zaid, the attorney representing two men closely involved with Able Danger, and Rep. Curt Weldon, R-Pa.

Weldon initially brought Able Danger to light about three months ago and has long been pushing for greater intelligence data sharing among the Defense Department, CIA and FBI. In a June speech before the House of Representatives, he expressed dismay that Able Danger did not appear in the 9/11 Commission's report.

An audibly frustrated Weldon praised the effectiveness of the Army's data-mining tactics, which have since spread to law enforcement agencies. He described a situation in which he was able to get a greater volume of information about a Serbian individual involved in the Kosovo peace talks through the Army than through the CIA.

Zaid said his clients, seated behind him, would have told senators that the Able Danger project did identify four Sept. 11 hijackers, including Atta, in its extensive data-mining process. All the information came from sources such as Lexis-Nexis, Westlaw, other subcontractors and the Internet--not government databases or classified sources, he said. His clients made repeated attempts to set up meetings with the FBI to share their charts and data, Zaid said, but all of those commitments were ultimately cancelled.

But Zaid was quick to note that the people on the charts could have been "nefarious or innocuous," and that "no information obtained at the time would have led anyone to believe criminal activity had taken place or that any specific terrorist activities were being planned."

Erik Kleinsmith, a former Army major who now works for Lockheed Martin, said he was ordered in 2000 by an Army lawyer to destroy all the data used in Able Danger. The reason for the destruction, he said, was an Army regulation that prohibits retention of data about U.S. persons beyond 90 days unless it has been determined to fit into one of 13 categories related to counterterrorism investigations.

The rules in question were issued by the president--not Congress--in the early 1980s in response to prior misuse of intelligence to gather information on American civil rights protesters, anti-Vietnam War demonstrators and other activists, said William Dugan, the sole Defense Department representative who spoke at the hearing. "We place special emphasis on the protection of information about United States persons," he said.

But, Specter asked, was Mohammed Atta considered a U.S. person?

"No," Dugan replied, after some prodding.

Dugan acknowledged that the regulations probably would have permitted the information in question to be shared with law enforcement agencies before its destruction. "Why wasn't it done in this case?" he asked. "I can't tell you."

Sen. Jeff Sessions, an Alabama Republican, suggested that a legislative remedy may be in order. "If a lawyer was too aggressive in requiring deletion of things they shouldn't have," he said, "Congress should look into that."

Specter indicated that additional hearings may take place. "We are not dealing with a matter of minor consequence," he said, instructing Dugan to "talk to the (Defense) Secretary and tell him the American people are entitled to some answers."