An Ontario fire marshal investigator says the victims of a deadly seniors' apartment building fire in east Toronto may have been overcome by smoke after fleeing their apartments and venturing into a top floor hallway.

Three people died and 15 more suffered smoke inhalation or other minor injuries after a fire broke out on the fifth floor of a Toronto Community Housing building at 1315 Neilson Rd. in Scarborough.

"It's my understanding that people went from a position of relative safety into the fire area," fire marshal investigator Jeff Tebby told reporters outside the building on Friday night.

The fire started on the fifth floor just after 3 p.m. and firefighters who arrived quickly called for more trucks as thick black smoke filled hallways and stairwells. Firefighters ordered those in the building to take shelter in their apartments.

"The most difficult part was the extent of the smoke," Toronto Fire Services Cmdr. Bob O'Halloran told reporters. 

Eventually, firefighters had no choice but to move some fifth-floor residents out of the building by carrying them down fire truck ladders, O'Halloran said. 

Toronto Community Housing seniors apartment fire Neilson Road

A woman who suffered smoke inhalation, according to her son, is loaded into an ambulance. (CBC)

Toronto Paramedic Services said four people without vital signs were taken to hospital, and three of them were later pronounced dead. Others were also treated for smoke inhalation or minor injuries.

"It's a real tragic thing," local city councillor Raymond Cho told CBC News at the scene, where he rushed after hearing about the fire.

Currently, it's unclear what caused the fire, but the Ontario fire marshal's office, which investigates all fatal fires, has already said the building had sprinklers and working fire alarms.‚Äč

Tebby said the victims' ages weren't necessarily to blame, but poor fire safety education may be a factor.

"You need to have a plan if there's a fire in the building," he said.

Son calls blaze 'scary'

Dozens of people, including some concerned family members, gathered outside a cordon that police set up near the building as firefighters carried out the evacuation.

Mohan Panchalandan told CBC News his mother lives in the building on the fourth floor, just under where the fire broke out.

TCH fire

A fire truck's ladder extends to the window of the Toronto Community Housing seniors' residence, where a fire broke out Friday afternoon. (Vedran Lesic)

"When the fire started happening they couldn't get out," he said. After multiple attempts he was able to reach his mother by telephone to confirm she was safe and taking shelter on the first floor, but "very upset."

Jonall, who provided only his first name, watched his mom get loaded into an ambulance after she was removed from the fifth floor. She's suffering from breathing discomfort, but should be OK, he said.

"It is scary, yeah," he said of the fire.

Jonall said his mother smelled smoke and heard alarms ringing in the building.

Other residents who were pulled from the building were wrapped in orange blankets before being taken to paramedics and TTC buses. 

Residents on the first to fourth floors have been resettled, but it could be some time before those on the fifth floor can return to their apartments. 

Building relatively new

Mayor John Tory, who was in Ottawa for meetings on Friday, tweeted that he was "saddened" to hear about the fire.

"Thoughts are with those injured, the families [and] all those affected," he wrote, adding TCH staff are on scene to help.

The building has about 125 units, and many of the residents are of Punjabi and Tamil backgrounds, Cho said. 

Gary Anandasangaree, the area MP, said he had recently visited the building and frantically called those he knew there after hearing about the fire.

Anandasangaree said the building was built in the 1990s and is well maintained. He said he believes it's a good place for those who live there and nurses often visit to support the seniors with health needs.

"This is tragic," he told CBC News.  

"We just need to make sure that we support the people who are here and give them the necessary assistance." 

More than a dozen trucks responded to the fire.