Yeah, that's not how it works. The 3 known people would have been arrested and charged. No way that they wouldn't squeal on the other two people, either out of just being an idiot and talking to the cops, or as part of some deal offering. They don't just wait for every single person to be ID'd before doing anything at all, especially in a gang rape case which could potentially be charged as a capital crime (unlikely but possible).
But I will say yes, you could be right. Not really saying you're not, just that with my time in the CJ world, that's been my experience with how things usually develop. Either way, awful as hell all around.