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The Baton Rouge Fire Department, hazmat and EMS are on scene of a fire at the ExxonMobil refinery on Scenic Highway, Tuesday, November 22, 2016, in Baton Rouge, La.

A fire at the ExxonMobil refinery that injured six workers on Nov. 22 also released 600 pounds of isobutane into the air, but the highly flammable gas vapor never escaped the huge facility's site in north Baton Rouge, the company reported.

ExxonMobil officials told the state Department of Environmental Quality Tuesday that they don't yet know if the 14-minute-long fire and release were preventable, but is continuing to look into the incident as the U.S. Occupational Safety and Health Administration, and U.S. Chemical Safety Board have opened their own investigations.

Since 2008, the 2,100-acre refinery and chemical complex just north of the State Capitol has drawn sanctions from the U.S. Environmental Protection Agency, DEQ and OSHA over safety, air quality and reporting failures, agency records show. But company officials maintained Wednesday that overall air emissions for key pollutants have significantly dropped since 1990 while workplace incidents have fallen 60 percent over the past five years.  

"We are cooperating with all agencies involved, and any additional information will be provided when the investigation is complete," said Stephanie Cargile, a refinery spokeswoman. "Investigation findings will enable us to take corrective actions and apply preventive steps that are identified."

OSHA investigators remained at the refinery Wednesday, an agency spokesman said.

ExxonMobil officials told DEQ in a required report Tuesday that the isobutane vapors escaped from its alkylation unit and sparked "a combustion incident" at 3:56 p.m. Nov. 22. The fire and release were contained and halted by 4:10 p.m., the company said.

The unit is part of the refinery's "light ends complex," which was undergoing maintenance at the time. The unit mixes isobutane with olefins and a sulfuric acid catalyst to make alkylate, a component for high octane gasoline, DEQ permits say. 

Isobutane is a colorless liquefied petroleum gas created during oil refining and is extremely flammable. Its heavier-than-air vapors can travel along the ground to reach distant ignition sources, material safety data sheets say. 

Among the six people injured in the fire, four remained Wednesday in Baton Rouge General Medical Center's Burn Unit. Meghan Parrish, hospital spokeswoman, said three people have been upgraded from serious condition to fair. The fourth person remains in serious condition.

The other two people hurt in the fire had minor injuries only requiring first aid, Cargile said.

ExxonMobil's technical estimates show the isobutane vapor release was small enough that the refinery would not have been required to report it to DEQ had there not been a fire with injuries, the company told DEQ.

"There were no offsite impacts as a result of this incident," ExxonMobil reported. "Air monitoring at the facility fence line was conducted, and all results were below detectable limits."

DEQ staffers using hand-held monitors found similar results the day of the incident.

ExxonMobil had initially believed more material was released, but it still wasn't finding offsite impacts. Company officials told Louisiana State Police in the first hours after the fire that as much as 2,500 pounds of butane and isobutane had been released, a State Police incident report says.

The company's required report to DEQ Tuesday is the most detailed explanation ExxonMobil has provided so far about the fire inside the nation's fourth largest refinery.

Citing company policy last week, company officials refused to disclose what unit burned — though DEQ had — or what may have been released into the air, while also saying there was no offsite impact and that production at the 503,000 barrel-per-day refinery was not interrupted.

Even with the report to DEQ Tuesday, details are still emerging on the cause of the fire.

The Chemical Safety Board reported last week that the fire started during "unplanned maintenance around a pump."

But Cargile said Wednesday that the "incident occurred during planned, routine maintenance activities."

DEQ records show the company had been doing major maintenance, known as a "turnaround," on the alkylation unit since Sept. 27. Less than two weeks earlier, DEQ had granted a minor change to ExxonMobil's air permit for the unit and the rest of the "light ends complex" in anticipation work on flare systems required by new air quality rules, permit records show.

Then, on Oct. 27, the company told DEQ that reconstruction of the "C-1" compressor in the alkylation unit was starting as a part of the ongoing turnaround. 

Last week, Reuters, citing anonymous sources, reported that the fire broke out as five people were preparing to restart a compressor following repair work.

Cargile said Wednesday it was too early to speculate on the fire's cause. ExxonMobil officials did tell State Police the night of the incident that they believed a six-inch valve in a compressor blew out and released the vapor, the State Police report obtained Wednesday says.

The Chemical Safety Board, created to ferret out the cause of chemical industry accidents, sent investigators to the Mississippi River refinery last week, agency officials said. 

The investigators' arrival marked CSB's second visit to Louisiana's petrochemical corridor in a bit more than a month.

Agency officials were in Baton Rouge Oct. 19 to announce the findings of its three-year investigation to the deadly Williams Olefins fire in Geismar in nearby Ascension and Iberville parishes. The agency found more than a decade of safety lapses led to the blast. 

Follow David J. Mitchell on Twitter, @NewsieDave.