After the teens got into a fight, the nude photos they'd taken of each other suddenly became bargaining chips.

Threats followed, and ultimately, one 13-year-old's photographs were posted on the Internet.

Increasing online sexual extortion, or "sextortion" cases, are worrying safety advocates and causing some frightened victims to give in to demands such as posing for explicit photos or having sex with the perpetrator.

Teen girls are the most- common target, said detective Rob Kemmet with the Oklahoma City Police Department's Predator Interdiction Unit, but it can happen to almost anyone.