The Thunder Bay Fire Service is investigating a house fire at 202 Rowand St. that happened overnight.

Firefighters were called to the vacant house at around 1 a.m.

Fire investigator Bob Vander Ploeg said they don't yet know where or how it began.

"We're looking at a fire that, when fire crews arrived, was showing on the exterior of the east wall of the building, and also on the interior," he said.

"So at this point, we're trying to determine whether the fire began on the exterior and burned into the house, or started inside the house and burned out." 

Two neighbouring buildings were at risk, so firefighters evacuated a home on one side and an apartment building on the other.

A total of 12 people had to stay out of their homes for a couple of hours until the fire was under control. A city bus was brought in to keep people warm.