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 examples victimizing 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 Alabama, Pennsylvania and Missouri.

-- 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 phone photos, which he then used to extort them for sex.

-- A 31-year-old California man was arrested in June on extortion charges after authorities said he hacked into more than 200 computers and threatened to expose nude photos he found unless their owners posed for more sexually explicit videos. Forty-four of the victims were juveniles, authorities said. Federal prosecutors said he was even able to remotely activate some victims' webcams without their knowledge and record them undressing or having sex.

The cases have prompted law enforcement officials and advocates to caution teens about their activities. Privacy is nonexistent on the Internet, and once indiscretions appear online, they are virtually impossible to take back. A nude photo sent to a boyfriend's cell phone can easily be circulated through cell phone contacts and wind up on websites that post sexting photos. Once there, it's available for anyone who wants to trace it back to the person who made it.

"Kids are putting their head in the lion's mouth every time they do this," said Parry Aftab, an attorney and online child safety advocate.

Teens can be more vulnerable to blackmail because they're easy to intimidate and embarrassed to seek help. And the extortionists are often willing to make good on their threats, said Steve DeBrota, an assistant U.S. attorney in Indianapolis who has been involved in sextortion investigations.

"You are blackmailable," said Aftab, " ... and you will do anything to keep those pictures from getting out."

In the Indiana case, the teenage girl's mother called police when she found out about the threats. Authorities subpoenaed Internet service providers to track the chats and e-mails to their source, a computer in Mechanicsville, Md., according to court documents.

According to court documents, the computer's owner, Trevor Shea, told agents he had engaged in similar schemes with about 10 girls, most of them 17 or 18 years old.

His trial is set for Aug. 30. He has pleaded not guilty and his attorney, Michael Donahoe, said he is working on a possible settlement.