The Perfect Locked-Room Murder? 🕵️♂️

In the dead of winter, a person was found stabbed to death inside a warm, heated, locked room. 🔍The room's windows and doors were all securely locked from the inside, and there were no tools or footholds within the room to pick the lock, nor any trace of anyone leaving. 🔍Furthermore, thick dust had settled evenly on the door cracks and window sills, indicating that any entry or exit would have left a mark. 🔍Aside from the victim's wounds, there were no signs of a struggle inside the room. The police ruled this case as 'suicide' rather than homicide. How could this have happened? 💡 Hint: Consider the room's 'building materials' or seasonal factors.