Read the excerpt from a short story.

The apartment house had a communal attic rarely visited by its tenants. The labeled boxes and cast-off furniture offered little intrigue, so few explored its depths. Then one rainy afternoon, I ventured there seeking solitude.
Roaming beyond the beams of the lone ceiling bulb, I spied only cobwebs until I rounded the corner of a hulking armoire. Beyond this antique was unrelenting shadow, but I willed myself to advance. Then a grim bellow of thunder resounded, sending me to the floor. Prostrate on the splintered beams, my hands felt a shaft of cold steel and I could but imagine the evil article I'd encountered.
What technique does the author use to build suspense in the excerpt?

The author uses foreshadowing and vivid descriptions to build suspense in the excerpt. The mention of the "unrelenting shadow" beyond the antique armoire and the discovery of a "shaft of cold steel" create a sense of anticipation and tension for the reader. Additionally, the use of a grim thunderous sound adds to the eerie atmosphere, heightening the suspense and building a sense of foreboding.