Understanding the difference between inner dramatic tension, which is character-driven, and outer dramatic tension, which is plot-driven, helps demystify the process of fiction writing. In writing your scene, create an interplay of both.
This mysterious picture immediately sets up visceral tension. Which character has the point of view? The killer? The detective? The unknown person behind the window?
What if the detective doesn't know if the person behind the window is the killer or the victim?
Or does the point of view belong to the terrified victim?
Now go beyond the picture and get inside the mind and gut of either the killer, the detective, or the person behind the window. This creates inner dramatic tension and deepens the page-turning quality of your scene.