Tesoro Corp. is under fire from federal officials after refusing to cooperate with an investigation into a chemical release that seriously injured two workers at a Northern California refinery.

Two plant workers suffered first- and second-degree burns in the Feb. 12 incident, when their bodies were sprayed with sulfuric acid while putting a sampling station back in service at the refinery. The unit was shut down after the incident and will remain shut down until the California Division of Occupational Safety and Health issues, or Cal/OSHA, and approval to restart.

In a letter from U.S. Chemical Safety and Hazard Investigation Board, federal officials claim Tesoro officials haven’t allowed the agency to return to the site to conduct interviews, did not preserve the accident site, and have not given access to requested documents related to the incident.

“We have determined that a mechanical integrity failure occurred on equipment connected to a 100,000 gallon process vessel containing flammable hydrocarbons and concentrated sulfuric acid,” the letter said. “We have also learned that protective equipment required by procedure for sampling was not provided for workers at this time.”

Company officials said the incident is under Cal/OSHA’s jurisdiction, and in a statement Feb. 21, Tesoro spokeswoman Melissa Flynn expressed surprise that the CSB wanted to investigate incident, she classified as resulting in “minor chemical burns.”

“Tesoro has decided that it must respectfully decline to participate in the CSB’s review due to their lack of authority in this particular instance,” the statement said.

Board members from the CSB refuted the claim that the injuries were minor.

“Acid splashing on workers’ unprotected faces or other parts of the body, resulting in first- and second-degree burns requiring air evacuations to a hospital burn unit, treatment, and subsequent significant lost time at work, absolutely constitute serious injuries,” CSB officials said.

The chemical refinery injury attorneys of Schechter, Shaffer & Harris, L.L.P., Accident & Injury Lawyers  represent plant workers injured on the job and the families of workers killed in refinery accidents.