There are actually two murders in this book. The neighborhood in which April is living is home to many children, some of whom April has bonded with. It is not any of April's friends, but two other girls in the neighborhood, that have been murdered. The suspect, until he saves April from the murderer, is the Professor who lives on Orchard Street.
Actually It is not the Professor. The Professor was the one who actually saved April from being tooken away/hurt. Why would someone that wants to murder April, Save her? That doesn't make sense. It's possible but no. The real murderer is Mr.S's Relative. When Mr.S rushes his customers and puts children last, what would you say that means? If Mr.S has a relative that could possibly work or be with him, he may tell Mr.S to put children last so he can actually get to one of them. Oviously, he got to 2 of them (little boy and the girl murdered and found by the bay). Also, a suspect COULD be Professor but it's not him. Another one could be the Janitor. The Janitor lives right near Elizabeth. When the two girls, Melanie+April, went to pick her up for her first day of school, the Janitor made silly jokes with them. Usually, when you make jokes with a child, it's your way of being too nice to get them into Closer to you, causing some kids to get killed. I wouldn't just come out and say who the murderer was because just like I wanted to know and found out, you will too. That's enough info. Thank-you.