The Ontario Fire Marshal is investigating the cause of a massive fire in Bolton that gutted six townhomes early Wednesday morning.

Officials have determined that the fire on Alderbrook Place, close to Highway 50, began on the backyard deck of a townhouse before spreading to the other units through the attics.

Three of the units consumed by the fire have partially collapsed. The Ontario Provincial Police said the homes will likely have to be demolished after the intense heat caused extensive damage.

Resident Elizabeth Nicholson said she managed to get out of her home with just the clothes on her back.

"I was asleep and the next thing I know I hear somebody calling my name," Nicholson said. "It's my neighbours from the house next door. They were out on the driveway yelling at me that there's a fire."

Nicholson is staying with a neighbour.

While all the homes had working smoke alarms, police said they didn't sound immediately because the flames were travelling through the attic. Most residents were alerted to the fire by their neighbours.

"There was a red light from the fire shining through my window," Nick Taccogna told CTV Toronto. "It sounded like rain so I didn't really know what was going on. Then someone came to the door and that's when we all had to get out of the house."

More than 55 firefighters rushed to the early-morning blaze, which began around 3:30 a.m. By the time fire crews reached the scene the fire had ripped through the adjoining houses. The blaze was largely under control by 8 a.m.

With a report from CTV Toronto's John Musselman