Investigators from the federal Chemical Safety Board have begun work to determine the cause of a fire at Woods Cross' Silver Eagle Refinery in Utah, US, on 12 January.
The fire occurred inside a 440,000-gallon storage tank containing light naphtha.
The blaze engulfed two refinery workers and two contractors in a shed about 200 feet from the tank. The fire also forced the evacuation of 100 homes within half a mile of the refinery.
The fire burned for several hours, until the South Davis Metro Fire Agency extinguished the flames on the morning of 13 January.
A large vapour cloud was released from a petroleum storage tank, the Chemical Safety Board comments. The cloud was ignited by an as yet undetermined ignition source, causing a massive flash fire.
The chemical-safety team will examine the tank's history and the integrity of the tank's seal, while also looking at the operation of the refinery and any recent changes that could help determine why the volatile hydrocarbons were released.