The offices could not be more generic. The building is nondescript. There is a sign outside the door, but they're planning to take it down.

But inside the Cyber Crimes Center in central Fairfax, investigators are cracking the most sensational, horrifying, gut-wrenching criminal cases involving children, pornography, predators and the Internet. As a result, thousands of people around the world are being arrested; many are going to prison.

The center is a state-of-the-art forensic computer lab run by the U.S. Immigration and Customs Enforcement agency to help investigate domestic and foreign crime conducted on or made possible by the Internet.

The "C3," as law enforcement officials often call it, doesn't focus solely on pornography. The agency, part of the Department of Homeland Security, also uncovered a global network of software pirates, tracks sales of illegal drugs over the Web and has helped prosecutors show how terrorists use the Internet.

In November, the center's director, Special Agent James Plitt, was one of six ICE investigators honored by the Justice Department for the success of "Operation Buccaneer," which broke up a ring of software thieves that began in Australia and spread to the United States, England and Finland.

"The work they do is heroic," said Ernie Allen, president of the National Center for Missing and Exploited Children in the city of Alexandria. "The Cyber Crimes Center is really ICE's point agency in battling the problem of child pornography. They've really been very aggressive, very dynamic and very committed. They're an integral part of everything that's being done to impact this growing phenomenon of child exploitation."

The ICE launched "Operation Predator" in July 2003 to target child porn distributors and users, people who smuggle children and those who travel abroad to molest children. In less than two years, more than 5,700 people have been arrested in more than 100 countries, with crucial support from the Cyber Crimes Center. Some 5,000 were apprehended in the United States; the others are being prosecuted in their own countries. The investigation is continuing.

"We're using the Internet to track sex offenders who prey on children," Plitt said. "One of the big things 'Predator' has done is send the message that protecting children is one of our high priorities."

Plitt, interviewed inside a bland conference room at the center, declined to say how many agents or support staff members work at C3 or how much funding ICE provides, in part because the agency wants to keep a low profile.

"We do a pretty good job with the resources we have," he said. He noted that the FBI handles cyber-crime that does not have international links, but "any child exploitation case has probably touched somebody in this office" because those cases tend to have ties to other countries.

A prime example of how C3 reaches from Fairfax to the far corners of the globe is "Operation Regpay," which started with federal agents in New Jersey in 2003 and used C3's technical expertise. The goal was to find and arrest those who provide billing services for Internet child porn Web sites and profit from the sale of explicit images and videos.

A company in Minsk, Belarus, quickly became a key target and gave the effort its name. Called Regpay, it provided credit-card billing services for 50 child porn Web sites and hosted at least four Web sites of its own. After the operators were located in Belarus, the company's computers were seized and the Cyber Crimes Center "developed an overwhelming amount of leads" in tracking down subscribers worldwide, Plitt said.

The theory, or the fear, was that purchasers of child pornography could have been abusing children as well. And ICE spokeswoman Jamie Zuieback pointed out, "If you're buying the stuff, you're feeding the demand for more molestation."

In early 2004, as ICE agents fanned out across the country with search warrants, they found their fears often fulfilled: A Nevada man had videotape of himself abusing a young girl; a New Jersey man admitted he had molested or raped at least nine children; a Florida man was arrested while on his way to New Jersey to have sex with a 13-year-old girl.

Similar arrests occurred in England, Switzerland, Australia and New Zealand in the fall of last year and early this year. Australian authorities described their 200 arrests as the largest child porn crackdown in the country's history. More than 1,200 people have been arrested worldwide, and ICE officials have described Operation Regpay as the largest Internet child porn investigation ever conducted.

Allen, of the missing children center, pointed out that Regpay's Web site netted $3 million in less than a year, and C3's probe found it had 70,000 customers in the United States and another 26,000 worldwide paying $19.95 a month for graphic images of small children being sexually assaulted.

"If you had told me 10 years ago that there was a consumer market for this, I would've told you you were wrong," Allen said. "I would have said it's the province of a small, fixated group of pedophiles. What we have discovered is child pornography is a multibillion-dollar industry around the world."

Huge numbers of tips flow into C3, largely from the missing children center. Allen said the center received 112,000 child porn-related tips last year.

"It's a triage, like a battlefield triage," said Claude Davenport, the national program manager for C3. But, he added, "We have some of the most cantankerous bulldog investigators here," who will wade through the Internet sludge to track sellers and users.

The center also tries to identify and locate the actual children in the images to protect them from further abuse. Plitt said 85 percent of the child porn images on the Web are simply recycled from old photo shoots, and the victims have been identified. Courts often require that an actual victim be identified in the prosecution of a porn seller or user.

Lt. Kevin Butler, coordinator of an Internet Crimes Against Children task force in South Florida, said the Cyber Crimes Center's database of victims has provided "great evidence in court" for prosecutions in Florida. His investigators also consult C3 for analysis of evidence and for training.

"It's invaluable," Butler said of C3, "not only to the state of Florida, but to the whole world."

For more information, visit www.ice.gov.

Michael J. Garcia, left, assistant secretary for U.S. immigration and customs enforcement, accompanied by U.S. Attorney Christopher J. Christie, speaks at a news conference in Newark in January 2004, after more than 30 people were arrested in a case involving a Belarus-based child pornography enterprise.A display label in the lobby of the Cyber Crimes Center explains that "Information contained on a computer's hard drive can be transferred ('mirrored') to another computer to preserve the integrity of the original evidence while conducting the investigation." At left is a sign on the door of the center's state-of-the-art forensic computer lab; at right is the lab's network hub. The agency, part of the Department of Homeland Security, doesn't focus solely on pornography. It also broke open a global network of software pirates, tracks sales of illegal drugs over the Web and has helped prosecutors show how terrorists use the Internet. Interns work in the training room at the Cyber Crimes Center, or "C3," as it is known among law enforcement officials, a low-profile operation in a nondescript building in central Fairfax.