The city apparently has a volunteer course for Safety Rangers at Alexis, Nick and Ruby's middle school. It's generally finding lost toys or finding someone injured. It may be a license for hero duty, but is really just a school chore in the end, but it's not anymore when they find, instead of an autistic boy they were looking around for in the park:
1: A dead blonde girl their age that seems to have been strangled.
2: A footprint that got wiped off as other people found out about it.
3: A handful of other people who could've done it.
This is now a big deal, but like the general adult, the boss of these rangers tells them that the police will do their work on it. Meanwhile, after the incident, Ruby is told by her parents to quit the rangers because of the sudden danger, Alexis finds a crush, and together, they try to piece out why a Miranda Wyatt was murdered, and see if they can relate it to other murders that have apparently happened to "homeless" people. The story also goes into the lives of homeless people in general, being seen as unkempt thieves, others just having their phones and nothing else. Something handy for being a kid is Alexis had friends that knew Miranda (not official friends, anyway).
Another interesting thing is a side story of the killer doing his business whilst not showing himself off to us. Near the end, though, the mystery may give itself away a little too much, and the reason the killer is murdering people may sound weird to some, but at least he isn't doing it because he has to. When a murderer has to murder and is on the run, the good guys aren't supposed to catch him. If he's a lunatic, however, which he is, then by all means. This is a Book 1 in a new series called "Point Last Seen", but it doesn't end with a cliffhanger, so it doesn't commit you to the series but lets you decide if you want to see another intriguing mystery.