Work has resumed at an Oklahoma oil refinery where one worker was killed and another injured when a boiler exploded.

Sugar Land, Texas,-based CVR Energy Inc. said Sunday that maintenance work at the refinery in Wynnewood (WIN’-ee-wood) has resumed after Friday’s explosion that killed 34-year-old Billy Smith of Pauls Valley.

Employee Russell Mann of Davis remained in critical condition Sunday at an Oklahoma City hospital as a result of injuries he suffered in the blast.

CVR CEO Jack Lipinski said in a statement that the company is cooperating with state and federal investigators to determine the cause of the explosion.

Garvin County Sheriff Larry Rhodes says federal Occupational Safety and Health Administration officials arrived on the scene Saturday.

OSHA officials did not immediately return telephone and email messages on Sunday.