MARTINEZ  (AP) — Federal safety officials have been barred from returning to a Martinez refinery to investigate an incident in which two workers were burned by acid spewing from a broken pipe.

The San Francisco Chronicle says investigators with the U.S. Chemical Safety Board showed up a day after the Feb. 12 incident and gained access to the Tesoro refinery grounds. However, Tesoro has rebuffed the investigators’ subsequent requests to return.

Tesoro officials say incident was minor and did not qualify under the rules for a federal investigation.

The newspaper calls the challenge unprecedented. Safety board officials said they could not recall another refinery or chemical plant on U.S. soil that has challenged the board’s authority since its inception in 1998.

The workers were treated at a hospital for first- and second-degree burns and released the same day.