Carnivore or DCS1000
The US Federal Bureau of Intelligence (FBI) uses a small piece of software named DCS1000 or Carnivore.
The tool developed 1997-99, allows the FBI to intercept and monitor private email and Internet traffic secretly. Targets do not know that their computer activities are being monitored. Data intercepted depends on the type of warrant obtained by the FBI.
A terminal box loaded with the Carnivore software is installed on a rack next to an ISP's electronics and then attached to the network. The software acts as a packet sniffer, filtering all Internet traffic while it looks for certain data such as email messages.
The FBI has said that the tool would only be used to eavesdrop on suspects, not the general public.
Carnivore was implemented in October 1997 and replaced in 2005 with improved commercial software.