ATHENS, ONT. -- An investigation is underway following a fatal fire in Athens, Ont.

The blaze broke out at around 5:30 p.m. Sunday in the town northwest of Brockville.

Athens Township Fire Chief Mike Smid says fire crews arrived at the scene to find "very heavy fire and smoke conditions."

He says the house collapsed while fighting the blaze and a body was recovered the next day.

Ontario Provincial Police say an eight-year-old remains unaccounted for.

OPP say the name of the deceased will not be officially released while a post-mortem is underway.

Neighbour Bill DeJong says he was at home at the time of the blaze.

"I had gone into my shop for perhaps five minutes or so. When I had gone in, there was no evidence of anything amiss and when I came out, there was just black, billowing smoking coming out."

He says he ran to help, and found the mother outside.

"I emptied a couple of fire extinguishers and it was having absolutely no effect whatsoever. It was just too hot and too engulfed," he explained. "She attempted to go back to get her son and … I did prevent her from going back in. It would just have resulted in another death, I think."

He says it was a tragic moment.

"To have lost a child under such circumstances is awful," he said.

Smid said the fire was extinguished by 9 p.m. with the help of fire crews from neighbouring towns, including Elizabethtown-Kitley, Leeds and the Thousand Islands, and Rideau Lakes Township.

The Leeds County OPP crime unit and the OPP forensic team are assisting in the investigation, along with the Ontario Fire Marshal and the Office of the Chief Coroner.

Anyone with information regarding this incident should contact the Ontario Provincial Police at 1-888-310-1122.