A fire raged through a retirement home close to Brussels late Thursday, leaving nine people dead and three more in critical condition, officials said.

The fire broke out around 8 p.m. Thursday and quickly sent thick, black smoke through the sprawling facility where some 100 retirees receive care.

Officials said the fire was likely accidental.

"A piece of equipment overheated or imploded and very quickly there was dense smoke everywhere," said city alderman Frederik De Buck.

Most of the retirees on the wing where the fire broke out were already asleep, with their doors open on a day that temperatures topped 85 degrees Fahrenheit. Only a few on the wing who were still awake were able to get outside in time, De Buck said.

"It is especially the smoke that did it," said. "A lot of doors were left open because of the hot weather outside."

Some 30 people were treated for breathing difficulties after they escaped the blaze, said provincial governor Andre Denys. Two firefighters and two emergency workers were also treated for breathing difficulties.

Police said they were still investigating the cause before a definitive reason could be given for the blaze.

When emergency crews arrived, they had to treat many on the lawn outside. The local fire department called in help from the nearby city of Ghent to handle the blaze, which was quickly put out.

Melle, a city of about 10,000 people, is about 30 miles west of Brussels.