(Reuters) - CVR Energy Inc on Sunday said it launched an internal investigation into the cause of a boiler explosion at its Wynnewood, Oklahoma, refinery on Friday that killed one employee and critically injured another.

"Our focus is to determine how this accident occurred and what steps must be taken to avoid a repeat of this incident," CVR Energy Chief Executive Jack Lipinski said in a statement.

The explosion occurred on Friday at about 6:20 p.m. local time as operators were restarting a boiler that had been temporarily shut down as part of a turnaround process that allows for necessary maintenance and upkeep of operations.

The damage was limited to the boiler, CVR said. Process units and other areas of the facility were unaffected, and there is no evidence of an impact on the environmental.

Damage estimates were not immediately available.

(Reporting By Martinne Geller in New York; Editing by Maureen Bavdek)