Six members of a family, a couple and their four children, died in a fire that broke out in their home in the Belgian town of Ui on Tuesday night into Wednesday, local authorities said.

According to the first indications, the fire was caused by an accident and surprised the family who were sleeping at the time.

The children, four girls ranging in age from 5 to 14, all attended school and their parents were “actively involved in the social life of the community,” Mayor Eric Dawson said. “I’m devastated, so many dead from a fire, I haven’t seen anything like this in 30 years,” he added.

This tragedy has plunged this Walloon city, between Namur and Liège, into mourning.

The fire appears to have started where a sofa was located and the toxic smoke released when it burned proved fatal for the family, a spokeswoman for the Liege prosecutor’s office said. It may have been caused by a short circuit or by a candle or lit cigarette. Someone opened a window, letting air into the house, which fueled the fire.

Firefighters were alerted shortly before midnight on Tuesday, rushed to the scene but were unable to enter the home, which was already engulfed in flames. “When we arrived, the facade had already turned into a wall of fire, the flames were embracing the building from the ground floor to the second floor,” explained their leader, Christoph Zando.

According to Zando, the home did not have smoke detectors.