The fire began around 4:30 p.m. in the gymnasium section of the recreational facility on 96 Ave.
Fort St. John Fire Department training officer Dan Golob said when crews arrived, smoke had already spread through the building.
"There was heavy smoke in the lobby area of the North Peace Arena and heavy smoke coming out of the gymnastics gym on the west side of the building," said Golob.
There were some staff members in the building Saturday afternoon and the swimming pool was open, but the entire recreation complex was evacuated once alarms started ringing.
"We sent in initial attack teams, which encountered heavy temperatured smoke and heat. We were there for quite a while as we progressed into the building and were able to confine it and then control it," said Golob.
"Our guys, when they entered the building, they experienced high temperatures. It was recorded [with thermal imaging cameras] that they were dealing with 800 degree temperatures as they moved forward into the building."
According to Golob, visibility was low when firefighters responded.
"The crews were able to get some water onto the fire and after a short period of time we were able to make headway and protect the other two buildings that were attached to the gymnastics building," he said.
Golob said it was too early to say how badly the fire damaged the recreation facility, but he added there was some structural collapse.
The bulk of the damage was limited to a change room and office area outside the gymnasium., said Golob.
Nobody was hurt in the fire, and an investigator will work Sunday to determine the cause.