Bonita Springs, Fla --- Investigators want to know what caused a fire at the Bonita Springs Utilities wastewater plant on Wellfield Road. Four people suffered heat and chemical burns. Three victims had to be rushed to the hospital.

"The employees were using a device to measure waste water when there was a flash fire," said Bonita Springs Fire spokesperson Debbi Redfield.

The utility says a vendor was showing employees a new piece of equipment when something caused a sudden spark that ignited methane gas. The three workers and the salesman were burned. They are now all listed in good condition.

"We work really hard to have a safe environment," said utility director Fred Partin. "This was just a fluke accident."

Bonita Springs Utilities is conducting its own investigation and has called on the Occupational Safety and Health Administration for help.