OTTAWA -- The Ottawa Police Arson Unit and the Ontario Fire Marshal are investigating a Canada Day fire in Vanier that left one person dead, and at least 20 people homeless.

A man died in hospital after the fire in a 24-unit residential low-rise apartment building on McArthur Avenue.

Ottawa Fire responded to multiple 911 calls reporting fire and smoke visible at 359 McArthur Avenue at 9:45 p.m. Wednesday. When firefighters arrived on scene, they confirmed a working fire in the three-storey residential low-rise building.

Fire officials say one resident was found in a hallway and was transferred to Ottawa Paramedics for care. Paramedics said the patient was found in cardiac arrest, and was transported to hospital in critical condition.

Two other residents and a firefighter were treated on scene for injuries.

In a tweet, Ottawa Police says the Ottawa Police Arson Unit is assisting the Office of the Fire Marshal with a “fatal fire investigation.”

"We're called out at any point of time where there's a certain amount of loss or there's a fatal fire or if there's suspicion or deemed to be arson," said Bruce Graham, of the Ottawa Police Service Arson Unit.

The fire was under control around midnight. Ottawa Fire says 12 of the 24 units in the building are not habitable, leaving at least 20 people homeless. 

Corey Rankin was watching Canada Day fireworks from his balcony when he noticed sparks in a tree.

"I didn't really believe at first until I actually seen it, and then I seen people start freaking out and stuff like that and then I felt really bad," he told CTV News.

"I seen a little bit of a spark in the tree so when I seen the spark in the tree I focused in that direction. When I focused in that direction, I sort of seen a flame coming from the corner of the building and it went from the corner of the roof to the apartment."

The cause of the fire is under investigation. A damage estimate was not available Wednesday night.

Rankin says he hopes it wasn't started because of fireworks.

"If it was a firework—I'm not saying it was—but if it was, hopefully people don't shoot it beside buildings and houses and stuff like that because something like this could happen for sure."