A collision between two tanker trucks at an oil refinery in Wales killed both drivers and caused a massive explosion Thursday, police said. The explosion rocked the Chevron plant in Dyfed in southern Wales and the surrounding neighborhood, the Daily Mirror reported. The blast occurred at about 6:20 p.m. The exact number of casualties was unclear. While the Mirror put it at two, many news outlets said four were dead and one injured. Police called the explosion a \"tragic industrial accident,\" The Guardian reported. Greg Hanggi, general manager of the Dyfed refinery, promised a thorough investigation. He called the blast \"utterly devastating.\"