Law & Order: Special Victims Unit S06 E15 Hooked

"Hope in the darkness."

When the body of a teenager is found with her older cousin's I.D., Benson and Stabler investigate only to learn that that the teen had been part of a group who had 'hooked up' online. As their investigation intensified, they realised that the victim was the object of an older man's obsession, but before they can arrest him he also turns up dead.