Officials at Husky Energy are still trying to determine the precise cause of an explosion and flash fire that ripped through a natural-gas well approximately 60 miles west of Edmonton, Alberta, British Columbia, on Monday, March 7.

The exploratory well was being used to prospect for gas reserves through a technique called hydraulic fracturing, or “fracking,” in which fluids under pressure are injected into natural fissures within a gas formation to increase the rate of recovery. Thirteen contract workers suffered mostly burns; four workers remain hospitalized, one in serious condition.

Husky spokeswoman Carla Yuill said the fire at the well lease site was quickly contained and extinguished and the well shut down. She added that the facility produces “sweet gas”—gas without significant amounts of hydrogen disulfide—and as such poses no health dangers to the public.

“Our No. 1 concern now is the health and safety of our workers. We’re offering them professional assistance and counseling services. Our goal is to help them get through this.”

Yuill said she could not estimate when the well will return to operation. The incident is currently under investigation by Alberta Occupational Health and Safety as well as Husky’s own investigative team.