(CNN) -- Four people were killed and one seriously injured in an explosion at a Chevron Corp. refinery in western Wales, the refinery's general manager said early Friday. The blast at the refinery in Pembroke occurred at a storage tank that was undergoing maintenance Thursday evening, police and fire officials said. It was not immediately clear what caused the explosion, said Police Chief Superintendent Gwyn Thomas. The incident was under investigation. "This news is utterly devastating," the refinery's general manager, Greg Hanggi, said in a statement released Friday by Chevron. Hanggi described those killed and injured as contractors at the refinery. It was not immediately clear from the statement what the five contractors were doing at the time of the explosion. One contractor who was injured in the explosion was being treated for "serious injuries," Hanggi said in the statement. CNN's Matt Smith contributed to this report.
An explosion occurrs at a storage tank that was undergoing maintenance, officials say . Chevron's refinery manager identifies the dead and wounded as contractors . Authorities say it is not immediately clear what caused the explosion .