Carnivore is the software being used by the FBI in usa, which goes through all emails going through each and every gateway on the usa internet backbone. The software is designed to scan the details of the email and based on specific words and phases, identify emails which may contain information regarding any possible threat to "national security", etc.
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The word Carnivore is slang... the software and the boxes that handle this is now called DCS1000.
Again, one can always debate about the capabilities of the NSA and other agencies that deal with security enforcement... but I think what counts is the ability to handle volumes, and pin-point threats in real-time.
Trust me, such capabilities are a long way off even now.
Regards,
Amol Hatwar.