The injured employees were hospitalized with major burns covering most of their bodies. According to reports from the Associated Press, the injured employees are two men aged 34 and 41, and two women aged 29 and 36.
The blast shook homes as far away as five miles, and flames were visible from the plant as they shot into the sky.
The plant, owned by Tesoro Corp, was cited for multiple and serious violations by the Washington State Department of Labor & Industries just one year ago.
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The fire reportedly started at around 12:30 am Pacific Time, and several fire departments sent crews to battle the blaze. Workers were performing maintenance at the naphtha unit, the site of the explosion, at the time. The naphtha unit is involved in refining crude oil into gasoline and other fuels. Crews got the fire under control around 2:00 am Pacific time.
Tesoro is a Fortune 150 company based in San Antonio, Texas. The refinery located in Anacortes, about 70 miles north of Seattle, can refine roughly 120,000 barrels of crude oil per day, turning it into jet fuel, gasoline and diesel.