WOODS CROSS — A vapor cloud released from a petroleum storage tank ignited, sparking the flash fire earlier this month that burned four people at a refinery here and forced a mass evacuation of surrounding neighborhoods.

The U.S. Chemical Safety Board said Monday that its investigators are focusing on Tank 105 at the Silver Eagle refinery for the ignition source that sparked the Jan. 12 explosion and fire.

"The cloud was ignited by an as yet undetermined ignition source, causing a massive flash fire," the board said in a statement.

Two refinery operators and two contractors were standing in a shed about 230 feet from the tank. The board said they were engulfed by the flames and suffered serious burns. They are recovering at a local hospital.

The 440,000 gallon tank was almost full the night of the accident, containing what the refinery terms "light naphtha." The tank has an internal floating roof and seven vents on the top sides of the exterior roof.

"The CSB team will be examining a reported history of releases from the tank and the integrity of the tank seal," CSB Investigations Supervisor Don Holmstrom said in a statement. "We will also be looking at the operation of the refinery and any recent process changes to determine why highly volatile hydrocarbons were released on Jan. 12."

Investigators have been going over the burned out tank for the past two weeks. The fire prompted the evacuation of homes within a half-mile of Silver Eagle Refining and took firefighters more than 12 hours to put out. The refinery has said it is also conducting its own investigation into the cause of the fire.

E-mail: bwinslow@desnews.com