KARACHI At least four workers were killed in a boiler explosion at the state-run Pakistan steel mills Wednesday, AFP said. Three people died on the spot and eight were rushed to hospital after "a boiler at the steel-making department blew up early Wednesday", said mill Assistant Manager Mohammad Javed. Another worker died of his injuries later Wednesday and seven others were in serious condition, hospital officials said. Investigations were underway but it was too early to say what caused the explosion, officials said.