MIDDLETOWN, Ohio (AP) _ At least three explosions near the blast furnace of a steel plant injured 13 workers Tuesday, sending six of them to hospitals. One was in critical condition.

The six men who were most seriously injured were working on catwalks in the iron-making area when the explosions occurred at AK Steel Corp.

The cause of the blast was under investigation, company spokesman Alan McCoy said.

One employee was in critical condition and five others were in fair condition at hospitals. Seven others suffered minor injuries.

It was the fifth major accident at AK Steel in the past 20 months. Seven people died in the last four.

Two Occupational Safety and Health Administration investigators looking into an earlier accident were at an employees' union office outside the plant at the time of Tuesday's blast.