MARTINEZ -- Federal chemical safety officials say Tesoro Corp. is preventing them from investigating an incident at its refinery near Martinez in which two workers were burned by acid spewing from a broken pipe.

Three U.S. Chemical Safety Board investigators showed up a day after the Feb. 12 incident and initially gained access, but Tesoro's lawyers raised "jurisdictional challenges" and forced the investigators to leave, said Dan Horowitz, the CSB's managing director.

"We've certainly faced our share of jurisdictional challenges, but I can't think of another refinery or chemical plant that has taken a position that injuries aren't serious enough for us to investigate and that we lack jurisdiction," Horowitz said. "This is a new one to me."

Two workers suffered minor injuries Feb. 12 after an acid spill at the Tesoro Golden Eagle Refinery. The workers were airlifted to UC Davis Medical Center and treated for first- and second-degree burns and released, according to a hospital spokeswoman.

Contra Costa Fire District spokeswoman Lisa Martinez said the workers suffered burns after being exposed to a sulfuric acid.

The safety board was created in 1998, and has spent much of the past 18 months investigating the August 2012 fire at Chevron's Richmond refinery.

Tesoro spokeswoman Melissa Flynn released an email statement Friday saying the company was "surprised" that CSB intended to investigate the incident, which she called a "personal safety incident that did not result in serious injuries or substantial property damage."

"We recognize the important regulatory role of the CSB, and the company will voluntarily work with the appropriate regulatory agency in every instance," Flynn wrote. "However, 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."

Horowitz disagreed, saying the CSB responded the day after his investigators were denied further access by issuing a subpoena to force Tesoro to comply with the investigation and turn over records related to the accident. Tesoro must respond to the subpoena by March 7, Horowitz said.

"Any time you lose containment in a refinery and workers are exposed, the incident is serious on its face," Horowitz said. "Clearly, it's important to find out what's going on (at the facility)."

The CSB's investigation of the Chevron fire in Richmond has resulted in hundreds of pages of investigative reports and recommendations for sweeping changes in refinery regulations.

California's Interagency Working Group on Refinery Safety, convened after the Chevron fire, released its final report last week, calling for improved community emergency response systems; better coordination among regulators; strengthening enforcement tools available to regulators; and requiring refineries to implement inherently safer systems, recommendations that in part echoed those made by the CSB.

"The bottom line is if the recommendations in the report are implemented, worker and public safety will be improved," Assemblywoman Nancy Skinner, D-Berkeley, said in a prepared statement. "The recommendations include increasing fines on violations and better public notification when incidents occur."

When asked in a telephone interview Friday about Tesoro's refusal to cooperate with the Chemical Safety Board, Skinner said, "CSB was established by Congress to help improve safety as an independent watchdog. I cannot imagine that CSB would not have a legitimate role or right to work on the incident at Tesoro."

On Feb. 18, the state Division of Occupational Safety and Health (Cal/OSHA) ordered Tesoro to shut down the section of the refinery where the pipe was located pending further investigation. An agency spokeswoman wrote in an email Friday that Cal/OSHA discovered violations of state regulations for injury and illness prevention and the safe handling of hazardous materials at the unit.

Flynn, the Tesoro spokeswoman, said Cal/OSHA was permitted access because "it is clearly within their jurisdiction to investigate."

In order to reopen the alkylation unit, which adds octane boosters to refined gasoline, Tesoro must demonstrate to Cal/OSHA that a series of steps have been taken, including a complete review of operating procedures and engineering controls, refresher training for workers in the unit and a review of management of procedures, said Cal/OSHA spokeswoman Erika Monterroza.

"Cal/OSHA's investigation continues into the incident," Monterroza said Friday.

According to Cal/OSHA's order halting use of the unit, operators interviewed said they are "afraid" to operate the unit and that Tesoro is "cutting acid rates and not running the unit as it is designed to be run."

The latest incident comes on the heels of Tesoro agreeing to pay $472,000 in civil penalties for 35 alleged air pollution violations between 2009 and 2011 in a settlement with the Bay Area Air Quality Management District. In April 2010, an explosion at Tesoro's Anacortes, Wash., refinery killed seven workers.

The CSB investigated that incident, and issued a draft report last month criticizing the company's approach to facility safety.

Contact Robert Rogers at 510-262-2726 or rrogers@bayareanewsgroup.com. Follow him at Twitter.com/roberthrogers.