INDIANAPOLIS: The nightmare began with a party: three teenage girls with a webcam, visiting an internet chatroom and yielding to requests to flash their breasts.

A week later, one of the girls, a 17-year-old from Indiana, started getting threatening e-mails. A stranger said he had captured her image on the webcam and would post the pictures to her MySpace friends unless she posed for more explicit pictures and videos for him. On at least two occasions, the teen did what her blackmailer demanded. Finally, police and federal authorities became involved and indicted a 19-year-old Maryland man in June on charges of sexual exploitation.

Federal prosecutors and child safety advocates say they're seeing an upswing in such cases of online sexual extortion. They say teens who text nude cellphone photos of themselves or show off their bodies on the internet are being contacted by pornographers who threaten to expose their behavior to friends and family unless they pose for more explicit porn, creating a vicious cycle of exploitation. One federal affidavit includes a special term for the crime: "sextortion."

No one currently tracks the numbers of cases involving online sexual extortion in state and federal courts, but prosecutors and others point toward several recent high-profile examples victimising teens in a dozen states: In Alabama, Jonathan Vance, 24, of Auburn was sentenced to 18 years in prison in April after he admitted sending threatening e-mails on Facebook and MySpace extorting nude photos from more than 50 young women in three states.

In Wisconsin, Anthony Stancl, 18, received 15 years in prison in February after prosecutors said he posed as a girl on Facebook to trick male high school classmates into sending him nude cell hone photos, which he then used to extort them for sex.

The cases have prompted law enforcement officials and advocates to caution teens about their activities. Once indiscretions appear online, they are virtually impossible to take back. Teens can be more vulnerable to blackmail because they're easy to intimidate and embarrassed to seek help.