You would think, if someone was looking in the girls in their tents before this happened, he was someone the children trusted and knew. I would start there, with someone who the children knew and were fond of. How he could rape and murder three small girls with no one realizing it until morning is fairly simple, sadly. One, if they knew him and trusted him they wouldn't start screaming until he hurt them. Secondly, and this is even sadder, all he would have to do to shut them all up, would be to tell them if you scream I kill these two. If you scream I kill these two. And if you scream, I kill these two. So they stay quiet, until he kills the first and than he quickly kills the other two before they have much screaming time. Than he leaves. And probably the counselors believed some of the children just stayed up and were telling scary stories or something, and most children were with counselors in their tents so, they probably believe it's being handled by one of them.