An explosion at a Chevron oil refinery in Wales has killed four workers and seriously injured another.

Authorities said the blast occurred Thursday as maintenance work was being done on a 730-cubic-meter storage tank. There was no immediate explanation for the explosion, which sent billowing plumes of black smoke into the sky although firefighters quickly extinguished a fire.

The local police chief, Gwyn Thomas, said investigators will try to determine the cause of the explosion, but that it appeared to be “a tragic industrial accident.”

In March, Chevron agreed to sell the refinery and other properties to Valero Energy as part of a $1.7 billion deal.