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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 November fire at the ExxonMobil refinery in Baton Rouge that sent four workers to a hospital burn unit started after an operator disassembled a piece of equipment during maintenance, causing a release of flammable isobutane.

According to a report recently released by the state Department of Environmental Quality, the worker was trying to open a pipe, but the hand wheel on the valve did not work. Exxon spokeswoman Stephanie Cargile said she did not know why the operator was unable to use the hand wheel.

"Hand wheels may not respond when turned, and there are several alternative ways of opening and closing a plug valve," she wrote in an email.

"In this case, however, the assistant operator removed the support bracket with hand wheel and gear box still attached rather than just removing the hand wheel and gear box."

The disassembly caused a release of isobutane, which was ignited by welding equipment about 70 feet away. Four workers were critically burned, though all have since been released from the hospital.

DEQ spokesman Greg Langley said that while the state has not officially closed the file on the incident, his agency has not planned to take any action because the isobutane was contained and no chemicals were released into the environment.

The incident is also under review by the federal Occupational Safety and Health Administration, but as of Monday, OSHA had not yet released any findings.

In their report to the state, Exxon officials said they have installed signs to indicate bolts that secure support brackets.

Follow Steve Hardy on Twitter, @SteveRHardy.