Investigators say the fire was caused by a "failure of the secondary phase" of the structure's electrical system.

By: KDKA-TV News Staff

PITTSBURGH (KDKA) — The Pittsburgh Bureau of Fire’s Investigation Unit says it has determined the cause of last month’s fire at the Gulf Tower downtown.

READ MORE: Gulf Tower Evacuated Due To Possible Transformer Fire, Smoke Rises From Top Of Building

According to investigators, the basement fire that sent smoke plumes from the top of the skyscraper was caused by a “failure of the secondary phase” of the structure’s electrical system.

They add that Duquesne Light Company’s infrastructure had nothing to do with sparking it.

The fire was contained to the basement but caused smoke to fill the entire structure. It knocked out power to half the building and prompted a full evacuation.

Fire officials said the smoke poured from the top of the building because “a ventilation shaft forces it out” as part of the building’s design.

Stay with KDKA for the latest on this developing story.