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 cell phone 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 […]