The Mystery of the Yellow Room

Storyline: Joseph Rouletabille, a reporter for a local newspaper, investigates the attempted killing of Mathilde Stangerson, who uses the yellow room of the title as her bedroom. At the time of the revolver shots her room was locked and the windows were barred, but when her father enters after having forced the door, there is no-one there except for Mathilde. So who did it and how did he get away?