it belonged to lisa ziegert. lisa worked evenings at the store. during the day, she was a teacher zaid. >> drove in and really couldn't quite figure out why she would be there at that time we. she would be at the agawam middle school teaching. >> was that obscene or car? >> definitely odd, yes. >> sofia parked and went inside. >> the door was unlocked and the music was on, the lights were on. >> so that's weird. >> yeah. >> so maybe she had opened the store? >> maybe she forgot something there and, you know, maybe had a break between classes, or you know, whatever, something, maybe. >> she called lisa's name, no answer. went up and down the narrow aisles, past the coffee mugs, the porcelain knick-knacks, the music boxes and cards. >> i walked around the counter where the cash register was and her purse was there. so, then, i started thinking, that's odd. it's not. why would her purse be there and she's not? exactly. >> she checked the back of the store, saw one room was a mess. >> were you worried that you are going to look into one of these rooms

lisa ziegert, full of funk and laughter, at peace, at last. >> i suppose i want people not to think about the horrible thing that happened to her. i want them to think of her as smiling, smiling down on all of us and giving the joy that she shared when she was here. that is what i want. >> hi. >> hi! >> having fun? >> i'm craig melvin. >> and this is "dateline." >> she was a rare gem to have as a friend, and we were lucky to have been with her. >> a bright and beautiful teen invites a friend to sleep over. the next morning, she's dead. >> oh, my god! i don't feel a pulse. >> what in the world happened? how could this be? >> turned out this was no ordinary slumber party. >> we know from their phones that this was a planned event. they had discussed it. they took pictures. >> stories surfacing of a dangerous experiment. >> the main purpose of having the sleepover was to try it. >> an experiment that just might tempt other teens. >> we know it's entered our high school. >> why the clues in this mystery could save a life in your family. >> you have a lot of people watching "dateline" that nee