OKLAHOMA CITY (AP) - A spokeswoman for the U.S. Department of Labor says determining the cause of a refinery explosion that killed one worker and critically injured another in Wynnewood, Oklahoma, could take months.

Elizabeth Todd said Monday that investigators with the federal Occupational Safety and Health Administration are at the scene in Wynnewood 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 when a boiler that was being brought back online exploded Friday. The 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.

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