MARTINEZ  (AP) — Federal officials are firing back at a Northern California refinery after the company blocked them from investigating a chemical release that seriously injured two workers.

Board members of the U.S. Chemical Safety and Hazard Investigation Board sent a letter Wednesday to the chief executive of Tesoro Corporation expressing disappointment in the company’s refusal to cooperate with investigators.

The letter claims that company officials have not permitted the board to return to the site in Martinez  to conduct interviews or to have access to requested documents related to the Feb. 12 incident, in which two employees suffered first- and second-degree sulfuric acid burns.

“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.”

The company’s refusal is contrary to a federal statute governing access and cooperation by companies during a CSB inquiry, the board said.

The acid was leaking at a rate of about five gallons a minute, according to the board members, when it splashed on the workers’ unprotected faces and other body parts, requiring them to be flown to a hospital burn unit.

The board says the company’s efforts to stymie the probe raises concerns that it may be trying to hide other issues.

The company has contended that the incident wasn’t serious enough to warrant a CSB investigation.

Efforts to reach company representatives were not immediately successful.