An incident at the Great Lakes Works facility on Zug Island Friday night put 15 employees in local hospitals.
A steel mill spokesperson said the incident was not an explosion but that "the issue occurred at the D4 Blast Furnace involving the dust catcher."
Fifteen contracted employees were taken to local hospitals, officials say. Since then, 12 have been released and three are still receiving treatment, according to the spokesperson.
No U.S. Steel employees were injured.
Officials are still investigating the incident.