A vital tool...

NY Times, reporting on the NSA program involving wiretapping Americans without a warrant:

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.

(snip)

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.