LOS ANGELES--Kristie was 13 when she met the first of her four "daddies." She had run away from home in the Southwest, and friends introduced her to a tall, good-looking man, who said the red-haired teenager was sexy and had potential. Pretty soon, he had her prostituting herself on the streets of Las Vegas--and then Los Angeles, Atlanta, and Phoenix.

The trafficking in children for sex was once thought to be a problem beyond America's borders. But the FBI and the Justice Department have now started focusing intently on the issue--and what they've found is shocking. Thousands of young girls and boys are falling victim to violent pimps, who move them from state to state, which makes it a federal matter. The younger they are, the more they're worth on the street. "There is a greater and greater demand for younger and younger kids," says Ernie Allen, president of the National Center for Missing and Exploited Children. "America doesn't look. People are shocked and horrified when they hear these girls' stories. They say, 'That doesn't happen here. It happens in Thailand. Or the Philippines.' But once you start shining a light on it, you find it everywhere."

A University of Pennsylvania study from 2001 estimates that close to 300,000 children nationwide are at risk of falling victim to some sort of sexual exploitation. Outreach workers concur, saying that of the 1 million to 1.5 million runaway children in the country, about a third have some brush with prostitution. "When we began initiating investigations around the country," says Johnson, "we found it everywhere we looked."

On the "strolls" of Sunset Boulevard and Figueroa, in South-Central Los Angeles, girls step out from the shadows in tiny skirts and stiletto heels. Detective Keith Haight sizes them up. He doesn't bother stopping unless they look underage, but it's hard to tell nowadays. Haight has been working these streets for 25 years and has seen the girls getting younger. This drop in age is due both to the rise of the Internet, which provides ready access to child pornography, and to the fear of HIV/AIDS. "Back then, if you found a 15- or 16-year-old, that was a big deal. But now, they're 11 or 12," he says. "If you go to the bus station, you can see the runaways coming off the buses, and you can tell the pimps waiting for them."

"The average life expectancy of a child after getting into prostitution," says Johnson, citing homicide or HIV/AIDS as the main causes of death, "is seven years." Tom O'Brien, the criminal division chief of the Los Angeles U.S. attorney's office, describes a conversation he had with one 14-year-old prostitute who was testifying against her pimp. "I told her, 'When I was your age, I thought I'd live forever.' She looked me in the eye and said, 'Mr. O'Brien, I'll be dead before I'm 21.' "