WASHINGTON -- In the anxious months after the Sept. 11 attacks, the National Security Agency began sending a steady stream of telephone numbers, e-mail addresses and names to the FBI in search of terrorists. The stream soon became a flood, requiring hundreds of agents to check out thousands of tips a month.
But virtually all of them, according to current and former officials, led to dead ends or innocent Americans.
FBI officials repeatedly complained to the spy agency - which was collecting much of the data by eavesdropping on some Americans' international communications and conducting computer searches of foreign-related phone and Internet traffic - that the unfiltered information was swamping investigators.
Some FBI officials and prosecutors also thought the checks, which sometimes involved interviews by agents, were pointless intrusions on Americans' privacy.
As the bureau was running down those leads, its director, Robert S. Mueller III, raised concerns about the legal rationale for the eavesdropping program, which did not seek court warrants, according to one government official.
Mueller asked senior Bush administration officials about "whether the program had a proper legal foundation," but ultimately he deferred to Justice Department legal opinions, the official said.
President Bush has characterized the eavesdropping program, which focused on the international communications of some Americans and others in the United States, as a "vital tool" against terrorism.
Vice President Dick Cheney has said it has saved "thousands of lives."
But the results of the program looked very different to some officials charged with tracking terrorism in the United States. More than a dozen current and former law enforcement and counterterrorism officials, including some in the small circle who knew of the secret eavesdropping program and how it played out at the FBI, said the torrent of tips led them to few potential terrorists inside the country they did not know of from other sources and diverted agents from counterterrorism work they viewed as more productive.
"We'd chase a number, find it's a schoolteacher with no indication they've ever been involved in international terrorism - case closed," said one former FBI official, who was aware of the program and the data it generated for the bureau. "After you get a thousand numbers and not one is turning up anything, you get some frustration."
Intelligence officials disagree with any characterization of the program's results as modest, said Judith A. Emmel, a spokeswoman for the Director of National Intelligence's office. Emmel cited a statement at a briefing last month by Gen. Michael V. Hayden, the country's second-ranking intelligence official and the director of the NSA when the eavesdropping program was started.
"I can say unequivocally that we have gotten information through this program that would not otherwise have been available," Hayden said.
The White House and the FBI declined to comment on the program or its results.
Still, the comments on the NSA program from the law enforcement and counterterrorism officials, many of them high level, are the first indication that the program was viewed with skepticism by key figures at the Federal Bureau of Investigation, the agency responsible for disrupting plots and investigating terrorism on American soil.
All the officials spoke on condition of anonymity because the program is classified. It is coming under scrutiny next month in hearings on Capitol Hill, which were planned after members of Congress raised questions about the legality of the warrantless eavesdropping.
The program was disclosed last month by The New York Times.
The law enforcement and counterterrorism officials said no active al-Qaida networks planning attacks had been uncovered inside the United States by the program. "There were no imminent plots - not inside the United States," said the former FBI official.
Some of the officials said the eavesdropping program might have helped uncover people with al-Qaida ties in Albany, N.Y.; Portland, Ore.; and Minneapolis. Some of the activities involved recruitment, training or fundraising.
But, along with several British counterterrorism officials, some of the officials questioned assertions by the Bush administration that the program was the key to uncovering a plot to detonate fertilizer bombs in London in 2004.
The FBI and other law enforcement officials also expressed doubts about the importance of the program's role in another case named by administration officials as a success in the fight against terrorism, an aborted scheme to topple the Brooklyn Bridge with a blowtorch.
Some officials said that in both cases, they had already learned of the plans through prisoner interrogations or other means.