East Chicago Mayor Anthony Copeland says one firefighter was injured after being struck by a fire hose.
The building covering half of a city block houses a recycling business and an auto glass company.
Nearby resident Juan Puente says a fireball erupted from the recycling business.
Another nearby resident, Ron Nieblas, says he heard several explosions.
The fire produced black smoke that was visible for miles, drawing a large of onlookers from East Chicago, Hammond and other communities.
Copeland says the cause of the fire is under investigation.