Whos The Killer?

There was a 15 friends who sleepover in the house of their best friend but when they wake up they found that one of their friend is killed and theres note in all of their cellphone saying i will kill 1 person every 30 min, if you find me and kill me you will be all be save but if you lose you will all die. do you think that they will be able to survive, do you think they will find out who is the killer, or do you think they will all die but lets find out.