The Missing Piece

By chance, a local chap Gustavo the Toy Maker creates a puzzle picturing a local murder scene. There is an image of a person leaving the scene. Could that be the murderer? He leaves the box unattended in his workshop and when some hoodlums break in and steal his tools plus the box with the puzzle he contacts the police. The box has been dropped outside Jesper's school and he takes it home for his twin sister and himself to try and finish.
They discover they know the crime scene from pictures in the local paper but also that one vital piece is missing; the one with the image of the suspected murderer's face. They decide to try to discover who the murderer is. They have the whole long summer holiday ahead of them with nothing much planned. So why not investigate? What could possibly go wrong?