Several hundred people were evacuated from the 36-story One US Bank Plaza building after explosions occurred in the street outside just before 5 p.m. The building is across from the St. Louis Convention Center, and construction work has been underway in the area for a couple of months.
The cause was not immediately known, but Fire Capt. Daniel Sutter said it originated in an underground vault that houses an electrical transformer. Sutter said the initial suspicion was that the transformer caught fire.
Crews from utility company Ameren Missouri were working to cut off electricity at the site of the hole in the pavement so that firefighters could extinguish the blaze.
Occasional fireballs and heavy smoke rose from the hole just a few feet from the edge of the building, which crews were hosing down.
"Firefighters are applying water to the building in order to protect it from the heat," Sutter said.
Hundreds of people were still at work inside when the initial explosion occurred. Besides bank employees, the building houses the offices of lawyers and other professionals. Many stood and watched the blaze after the evacuation.
Sutter said there was no apparent damage to the building, but it wasn't known if it would open fully on Wednesday.