"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, current and former officials say, 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."