MARTINEZ (KCBS)— Federal investigators looking into last Wednesday’s industrial accident at the Tesoro refinery in Martinez, where two workers were sprayed in the face with sulfuric acid, have been denied access.

On the morning of February 12th, two workers at the refinery suffered first and second-degree burns in a chemical release. They were flown to U.C. Davis Medical Center and released later that day.

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The San Francisco Chronicle reported that when federal investigators arrived the next day, they were allowed onto the grounds, but were denied return visits.

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On the day of the spill, refinery spokeswoman Claire Spencer told KCBS the area involved in the incident was immediately isolated and secured to ensure the employees were safe.

U.S. Chemical Safety Board officials said they can’t recall any other plant denying them access. Tesoro said it was a minor incident that should not prompt inquiry.

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The board has now subpoenaed documents about that unit’s operation.