A spokeswoman for the U.S. Department of Labor says determining the cause of an explosion at an Oklahoma refinery that killed one worker and critically injured another could take months.

Elizabeth Todd says Occupational Safety and Health Administration investigators are at the scene in Wynnewood (WIN’-ee-wud) Monday and could be there for up to a week.

Officials say the refinery owned by CVR Energy Inc. of Sugar Land, Texas, was shut down for scheduled maintenance Friday when a boiler that was being brought back online exploded. There was no fire and maintenance work at the plant resumed Saturday.

The blast killed 34-year-old Billy Smith of Pauls Valley and injured Russell Mann of Davis.

A spokesman at the University of Oklahoma Medical Center says Mann remains hospitalized in critical condition Monday.