Thunder Bay Fire Rescue officials say an investigation has determined that "vandalism arson" caused the blaze that heavily damaged a vacant house at Donald and Vickers streets on Sunday afternoon.
In a news release issued late Monday afternoon, Fire Chief John Hay asked anyone with information to contact Thunder Bay Fire Rescue or the police.
The Fire Service said a passerby called at about 3:20 p.m. Sunday to report the fire at 915 Donald St.
Platoon chief David Long said when firefighters arrived, there was heavy smoke around the roof and flames coming out a window.
Long said it took a while to bring the blaze under control.
"It would flare up, we had to get guys inside to try to find different spot fires," he said. "It was a good two to three hours before we really had a good handle on it to say that it wasn't going to ... ignite anymore."
Long said the two-storey house, which had no hydro or gas, suffered heavy damage.
This is the third fire to strike vacant buildings in Thunder Bay's south side in less than two months.
On Feb. 17, a fire gutted a building on Cumming St. Less than a week later, fire crews spent hours fighting a blaze blocks away on Finlayson St. Those incidents are under investigation. At the time, fire officials would not comment on whether they were believed to be related.