WASHINGTON, Jan. 16 - 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 F.B.I. 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, current and former officials say, led to dead ends or innocent Americans.
F.B.I. officials repeatedly complained to the spy agency that the unfiltered information was swamping investigators. The spy agency was collecting much of the data by eavesdropping on some Americans' international communications and conducting computer searches of phone and Internet traffic. Some F.B.I. 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 a program of eavesdropping without warrants, one government official said. Mr. Mueller asked senior administration officials about "whether the program had a proper legal foundation," but deferred to Justice Department legal opinions, the official said.
President Bush has characterized the eavesdropping program as a "vital tool" against terrorism; Vice President Dick Cheney has said it has saved "thousands of lives."
But the results of the program look 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 program and how it played out at the F.B.I., 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 F.B.I. 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 office of the director of national intelligence. Ms. 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 N.S.A. when the program was started.
"I can say unequivocally that we have gotten information through this program that would not otherwise have been available," General Hayden said. The White House and the F.B.I. declined to comment on the program or its results.
Notice the operative word "useful" missing in the last sentence.