From the New York Times,
2 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."