The Wisconsin State Journal reports that All Metals Recycling manager A.J. Vedvig said the explosion Wednesday happened in a scrap steel pile.
Vedvig says the scrap steel pile stands about three stories tall and contains between 20 to 30 semitrailer loads of material.
The recycling plant is about 2 miles south of the state Capitol.
The city’s fire department posted on Twitter that it was responding to multiple calls of an explosion shortly after 4 p.m.
Photos from the scene showed billowing black smoke and flames shooting into the air. Police and fire officials urged residents to avoid the area.