Installed on an Internet service provider's network, the system acts as a wiretap, filtering messages and storing information it deems relevant to its search.
Although Carnivore has been the target of a recent storm of media attention, the program has been used 25 times since its inception two years ago, Kerr said. The program has been used 16 times this year--10 times for national security matters and 6 times as part of criminal investigations.
Critics question whether the program violates Fourth Amendment search and seizure rights, as well as privacy rights.
The American Civil Liberties Union objects to the comparison of Carnivore to the traditional pen register, trap and trace device, or wiretap of a conventional phone line.
"Carnivore gives the FBI access to all traffic over the ISP's network, not just the communications to or from a particular target," said Barry Steinhardt, associate director of the ACLU. "Carnivore, which is capable of analyzing millions of messages per second, purportedly retains only the messages of the specified target, although this process takes place without scrutiny of either the ISP or a court."
In response to criticism, U.S. Attorney General Janet Reno has called for a third-party review of Carnivore's source code to verify the software's capabilities, said Kevin V. Di Gregory, deputy associate attorney general at the Department of Justice.
Recalling the covert surveillance of Martin Luther King Jr., Representative John Conyers Jr. (D-Michigan) voiced concern that this technology makes the most private conversations vulnerable to interception by government agencies.
"We have had many agencies, including law enforcement, go beyond the scope of their authority. It's nothing new? How do we convince people, 'It's OK. It's the government'?" Conyers asked.
"This system must not bite off more than it can chew when it comes to FBI Internet surveillance," Conyers said. "Our constitutional rights do not end where cyberspace begins."
Indeed, privacy advocates say Carnivore creates new threats to personal privacy online.
"Carnivore has access to much more information than it is legally entitled to collect. Yet there is little understanding of how monitoring is limited, and little chance for oversight. Such a situation is ripe for mistake or misuse," said Alan B. Davidson, staff counsel for the Centre for Democracy and Technology.
To mitigate these fears, during the hearing representatives from the FBI and Justice Department emphasized the narrowness of Carnivore's reach.
"Carnivore is simply an investigative tool that is used online only under narrowly defined circumstances, and only when authorized by law, to meet our responsibilities to the public," Di Gregory said.