Congress is hoping to get at the meat of the FBI's e-mail snooper, Carnivore — and decide if the surveillance system is a necessary law enforcement tool or a threat to privacy.

The House Judiciary committee held an afternoon hearing Monday with witnesses from the Department of Justice and the FBI, as well as privacy advocates wary of the new spy tool.

Carnivore, which the FBI discussed publicly earlier this month, is the name for a computer program set up at an Internet service provider to sift through e-mail and collect what is being sent to or from a particular criminal suspect.

"Constitutional rights don't end where cyber space begins," said Rep. John Conyers, D-Mich.

Dr. Donald M. Kerr, head of the FBI's laboratory division, which gave birth to Carnivore, testified that the technology is necessary to combat child pornographers, hackers who steal financial information, online con artists and terrorists. "Investigating and deterring such wrongdoing requires tools and techniques designed to work with new and evolving computer and network technologies," he said.

The bureau says the program is used only under court order and is closely monitored by officials in the U.S. attorney's office — and is less invasive than a telephone wiretap because it automatically reduces the amount of information coming in to the FBI through the filtering process.

But privacy advocates argue that Carnivore, in targeting crooks, could also ingest the communications of innocent citizens.

"To my knowledge, Carnivore is unprecedented in the history of domestic communications surveillance," testified Barry Steinhardt of the American Civil Liberties Union. "Carnivore is roughly equivalent ... to a wiretap, capable of accessing the conversations of all the phone company's customers."

Since 1980, federal wiretapping of telephones has skyrocketed by 230 percent. Last year, there were 1,350 court-authorized wiretaps, up 2 percent from 1998, according to a report from the Administrative Office of the U.S. Courts cited by the Electronic Privacy Information Center. No requests for wiretaps were turned down last year by any judge, the center said.

The FBI says its year-old Carnivore has been used less than 25 times and electronic surveillance has been effective in securing the conviction of 25,600 dangerous felons over the past 13 years.

And it's this point that should carry particular weight, said Dr. Amitai Etzioni of Georgetown University. "I'm in favor of privacy, but privacy always has to be balanced against public concerns," he said.

The FBI has been pushing technology companies to make their systems more compatible with government surveillance. In fact, the brouhaha over Carnivore began not because of a leak of classified bureau information but due to the FBI's own announcement about the system at a recent meeting of industry experts.

"They do have an obligation to be in compliance with the federal wiretapping statutes, so we are trying to help them along with that as best we can to make that as less burdensome for the industry as possible," FBI spokesman Paul Bresson said.

Some Internet service providers dispute whether the system is necessary at all, saying they can already produce the e-mails of criminal suspects if ordered to do so by a court.

"Reviewing all data to find some data is neither the most efficient nor the least intrusive method of electronic surveillance," said Peter Sachs, president of Connecticut service provider ICONN, LLC. "That is especially true when all ISPs ... can easily supply the FBI with all of the information it needs in a timely, accurate and efficient manner."

U.S. Attorney General Janet Reno said two weeks ago she would review the system to determine whether it might infringe on privacy rights, but Justice Department officials told the hearing on Monday it would not do so.

"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," said Deputy Assistant Attorney General Kevin DiGregory.

The ACLU last week made a freedom of information act request for the computer source code and other technical details about the system.

And the FBI says it will cooperate — to a point.

"There are some source codes they are looking for that obviously would not serve any purpose from our standpoint to give out," Bresson said. "If the world finds out how we created the system, it would make it easier for the bad guys to find out how to evade the system."

Rep. Conyers, the ranking Democrat on the full Judiciary Committee, said Congress needed to ensure Carnivore did not "bite off more than it can chew."

"Should we now be comfortable with a 'trust us, we're the government approach?'" he said. "I don't think anybody on this committee shares that view."

— Fox News' Rita Cosby and Reuters contributed to this report