WASHINGTON - The FBI has deployed an automated system to wiretap the Internet, giving authorities a new tool to police cyberspace but drawing concerns among civil libertarians and privacy advocates about how it might be used.
The new computer system, dubbed ''Carnivore'' inside the FBI because it rapidly finds the ''meat'' in vast amounts of data, was developed at FBI computer labs in Quantico, Virginia, and has been used in fewer than 50 cases so far.
But that number is sure to rise, said Marcus Thomas, chief of the FBI's cybertechnology section at Quantico. ''In criminal situations there's not yet been a large call for it,'' he said, but the bureau already has seen ''growth in the rate of requests.''
Civil liberties groups said the new system raises troubling issues about what constitutes a reasonable search and seizure of electronic data. In sniffing out potential criminal conduct, the new technology also could scan private information about legal activities.
''It goes to the heart of how the Fourth Amendment and the federal wiretap statute are going to be applied in the Internet age,'' said Marc Rotenberg, head of the Washington-based Electronic Privacy Information Center.
Across the Atlantic, the British government is seeking new authority to monitor all online activity - some of it without a warrant. It says it needs this power to combat terrorism, drug trafficking and pedophilia, but civil libertarians there are opposing the move.
The new American system, which operates on off-the-shelf personal computers, takes advantage of one of the fundamental principles of the Internet: that virtually all such communications are broken up into ''packets,'' or uniform chunks of data. Computers on the Internet break up e-mail messages, World Wide Web site traffic and other information into pieces and route the packets across the global network, where they are reassembled on the other end.
FBI programmers devised a ''packet sniffer'' system that can analyze data flowing through computer networks to determine whether it is part of an e-mail message or some other piece of Web traffic.
The ability to distinguish between packets allows law enforcement officials to tailor their searches so that, for example, they can examine e-mail but leave alone a suspect's online shopping activities. The system could be tuned to do as little as monitoring how many e-mail messages the suspect sends and to whom they are addressed - the equivalent of a telephone ''pen register,'' which takes down telephone numbers being called without grabbing the content of those calls.
''That's the good news,'' said James Dempsey, an analyst with the Center for Democracy and Technology, a Washington high-tech policy group. ''It is a more discriminating device'' than a full wiretap, he said.
But Mr. Dempsey expressed worries about the new system, which would be installed at the offices of a suspect's Internet service provider. Just as the device could be used to fine-tune a search, it also could used for broad sweeps of data. ''The bad news is that it's a black box the government wants to insert into the premises of a service provider,'' Mr. Dempsey said. ''Nobody knows that it does what the government claims it would do.''
Albert Gidari, a lawyer who works for the wireless industry, was present at a recent FBI demonstration of the system. He said the FBI's announcement was intended to counter industry assertions that it would be very difficult to provide the kind of pen-register wiretap capability that the agency wants.
Since the demonstration, Mr. Gidari said, one faction within the telecommunications industry was pleased with the FBI's efforts. But Mr. Gidari said the other faction was saying: ''Wait a minute - what are the liability issues? What are the privacy issues? We don't want third-party software on our system.''
Although Congress has passed legislation requiring telephone companies to make their developing high-tech networks easy to wiretap, Mr. Gidari is one of a large number of industry experts who believe the law does not apply to wiretapping the Internet. ''The FBI overreaches in everything they do,'' said Mr. Gidari, who is president of G-Savvy, an Internet consulting company.
A former federal prosecutor sounded a more supportive tone. ''If what it does is it helps comply with wiretaps, and it helps minimize what you're getting - to help get what the court authorizes you to get - there's nothing wrong with it,'' said Mark Rasch, now a security consultant with Reston-based Global Integrity.
Still, Mr. Rasch said the technology raised questions that have yet to be fully explored by law enforcement.
''The stuff that is examined only by a computer and not by a human being - was that information searched?'' Mr. Rasch asked. He then suggested an answer: ''It is a search, but it is to an extent less invasive than it would be if you did not use this technology.''
The first news of Carnivore actually came in April during congressional testimony by a Washington lawyer, Robert Corn-Revere, who represented an Internet service provider that tried to resist attaching the system to its network. Mr. Corn-Revere suggested that such a system could be used to track dissidents and journalists online.
But Mr. Thomas of the FBI said there was nothing mysterious about the new device. ''This is an effort on the FBI's part to keep pace with changes in technology, to maintain our ability'' to lawfully intercept everything from pen-register data to full wiretaps with court authorization.
