The collapse at the plant, located at 1577 Sparrows Point Road, was reported at 10:01 a.m., according to a statement from Baltimore County Police and Fire.
While workers were inside doing demolition work at the old cold strip mill, the roof collapsed, the statement said.
Four of the injured were in critical condition and were taken to the University of Maryland Shock Trauma Center, according to the report.
Five others were taken to local hospitals, officials said.
By Monday afternoon, three of the individuals at shock trauma were listed in serious condition and one was listed as critical, according to the report.
Approximately 65 personnel and 25 apparati from Baltimore County responded to the roof collapse, according to the statement. Two helicopters and seven medic units were used to treat and transport the injured, the statement said.
Baltimore County Police and Fire said all people at the plant were accounted for by 10:30 a.m.
Maryland Occupational Safety and Health was investigating what caused the collapse, county officials reported.