They Do It With Mirrors

In Agatha Christie's They Do It with Mirrors, the indomitable Miss Marple investigates some rather deadly doings at a rehabilitation center for delinquents.
Miss Marple senses danger when she visits a friend living in Stoneygates, a rehabilitation center for delinquents. Her fears are confirmed when someone shoots at the administrator. Although he is not injured, a mysterious visitor is less fortunate-shot dead simultaneously in another part of the building.
Pure coincidence? Miss Marple thinks not, and must use all her cunning to solve the riddle of the stranger's visit … and his murder.