Spy Agency Data After Sept. 11 Led F.B.I. to Dead Ends

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.

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