Investigators are trying to figure out the cause of a fire that ripped through several townhouses that were under construction in Innisfil.

Large flames tore through five units at Friday Harbour on Tuesday night. Officials say the fire had been burning for some time before Innisfil firefighters arrived on scene.

“It was a very difficult fire. The five townhomes were under construction with varying level of completion. Some units didn't even have drywall all the way up, so once the fire got into the wood it spread very fast,” says fire chief Jon Pegg.

The chief says crews went inside, but the stairway had burned down and the flames moved to the roof. The homes don't have all of their fire safety features built in yet, such as firewalls between units.

Firefighters were able to get the fire under control, but had to remain on scene overnight. A firefighter was injured, but has since been released from hospital.

Damage is estimated at $1.5 million.