You're not paranoid. Big Brother really can read your e-mail.

For a little more than a year, the Federal Bureau of Investigation has been using a computer system code-named Carnivore to read the e-mails of suspected criminals, the FBI said Tuesday.

Carnivore can scan millions of e-mails a second, and FBI agents have praised its crime-fighting power, but privacy advocates say the machine scares them because it could let the government spy on all online activities, from e-mail to banking and shopping.

The FBI revealed the system two weeks ago to "a roomful of astonished industry specialists," the Wall Street Journal reported Tuesday. The FBI says it has used the new snooping device in 45 to 50 criminal cases, mostly to track hackers, but also in counter-terrorism and drug-trafficking cases.

Carnivore, an off-the-shelf PC, is so named because it is designed to get at "the meat" of sought-after information. Agents take the machine directly to the offices of an Internet service provider (ISP). There they leave it in a locked cage, typically for about 45 days, making daily visits to retrieve captured data — e-mail sent to or from a suspect.

Like the more common phone tap, Internet taps must be authorized by court order.

Is Carnivore Too Hungry?

Critics say Carnivore gives the Feds too much access to private information.

"This is more of a vacuum-cleaner type approach — it apparently rifles through everything," David Sobel, general counsel for the Electronic Privacy Information Center, told FOXNews.com. "It's potentially much more invasive than telephone surveillance."

Carnivore in theory could process all the e-mail that passes through the ISP — not just messages sent to or from the suspect. Critics say that's like snooping on all the phones in a neighborhood to zero in on one phone.

One unidentified ISP put up a legal fight against Carnivore early this year but lost, according to the Journal.

The FBI says Carnivore is used much more surgically than that and only reads the e-mail of the target — and that messages belonging to those not being probed, even if criminal, would not be admissible in court.

"The volume of e-mail in a location is generally fairly small, and being managed by a small number of e-mail servers on a fairly low-speed network," said Marcus Thomas, chief of the FBI's cyber technology section. "The system is not unlike "sniffers" used within the networks every day."

But Sobel likened Carnivore to Russia's surveillance system, called "SORM," which all Russian ISPs are required to install in order to facilitate government snooping.

He also compared it to Echelon, the U.S. National Security Administration's system, which intercepts telecommunications transmissions from around the world and sifts through them for keywords that could describe illegal activity.

The American Civil Liberties Union was drafting a letter to Congress Tuesday demanding an investigation, an ACLU spokeswoman said. Sobel agreed that Congress should hold hearings on Carnivore to "ask some questions and get some answers," he said.

"Carnivore is really the latest indication of a very aggressive stance that the Bureau is taking in collecting as much information as technically possible," he said.

FBI spokesman Paul Bresson said law-abiding citizens would be protected, not violated by Carnivore. "Anytime we develop a system, we're basically balancing the interests of national security against that of the privacy of the public," he said. "This issue's always gonna come up, we're always gonna get questions, we understand that."