How...Why...What?

Could this really be happening????? My inner voice is asking "But How?". I think the one who called Maxine was really Derek's killer. I think this because in the book, it says that Maxine said the voice was a bit lower than her brother's actual voice. But then she said that she didn't even remember his voice. How creepy it would be to get a phone call from your dead brother! My inner voice is exploding, but I cannot jump to conclusions just yet. I need to keep reading and find out who actully called Maxine.