The deadliest refinery accident since 2005 was 'more of a fireball than an explosion' that killed five workers at the Tesoro refinery in Anacortes, Wash., according to investigators. A vapor cloud of hydrocarbons ignited within seconds early Friday morning, burning seven Tesoro employees who were working about 50 feet from a unit used to make high-octane fuel, said Robert Hall, lead investigator with the U.S. Chemical Safety and Hazard Investigation Board. Although residents near Tesoro's plant said they felt windows shake, the actual scene itself doesn't show significant blast damage, Hall said. The accident is the worst since 15 workers were killed and 170 injured at BP's Texas City refinery in 2005. The CSB sometimes makes urgent recommendations that may include a request that a plant close for a time, but the CSB isn't making that request of Tesoro, Hall said. CSB spokesman Daniel Horowitz said that after the BP disaster in Texas City many people outside the industry recognized that the safety of this sector needed to improve. The CSB oversees 150 refineries in the United States and thousands of chemical plants; about half of its outstanding investigations are of accidents at refineries, officials said. The CSB's investigation is expected to take about 18 months to complete, Hall said. The CSB is an independent federal agency that investigates major industrial accidents. 