A fire that killed four people in Oshawa, Ont. early Monday morning might have claimed more victims if not for the actions of a 50-year-old man, who ran into the flaming multi-unit home three times to save people trapped inside, his family says.

Steve Macdonald, 50, was seen by neighbours running back into the inferno he had already escaped to rescue others.

His family said Tuesday that some of them might not be alive if not for his fearless actions, which took his life. 

"He just wanted to get everybody he could out of that house," said Brandon Desroches, who spoke to CBC News on behalf of the household.

"Unfortunately he didn't even get all of our people out. He didn't make it. He forgot about himself."

Officials have released few details but neighbours and others say a family of four lived in the unit upstairs, which had a separate entrance. 

bonchek family fire

Lindsey Bonchek, 36, and her two children — Madeline, 9, and Jackson, 4, — also died in the fire. (Lindsey Bonchek/Facebook)

Lindsey Bonchek, 36, and her daughter Madeline, 9, died in the blaze that gutted the two-storey home. Bonchek's four-year-old son, Jackson, was pulled from the fire but later died.

Desroches lived in the basement along with Macdonald's daughter, Alysha. The two are engaged and expecting their first child in March.

Desroches was at work when the blaze began, but says five people were inside the Macdonald residence — the main floor and the basement — at the time. They included Macdonald's ex-wife Lauren, his girlfriend Sam and a family friend who had rented out a room in the basement.

Fire 'absolutely everywhere'

Alysha was in the couple's basement bedroom when "she heard her mother yelling and her father screaming," Desroches said.

By the time she reached the bottom of the stairs, there was "fire absolutely everywhere."

Desroches described a panicked Macdonald running downstairs to help his pregnant daughter up the stairwell and outside to safety. He ran back in for his ex-wife, and then ducked back in once more for his girlfriend, who was in the living room. (Early reports about the fire said he had gone back in to rescue children.)

But Macdonald didn't see the family friend outside after helping his girlfriend escape, Desroches said.

He entered again to search for her, and never re-emerged.

Eyewitness video of deadly Oshawa house fire1:04

The friend had already climbed out a basement window. "He didn't know that," Desroches said.

"He was just selflessly trying to get her out. And I know him, his next move would have been to bring the ladder we had on the side of the house, and try to get people out from upstairs. He was that kind of guy."

Other members of the family declined to comment on Monday's fire.

'We lost a true hero'

Desroches recalls Macdonald as a generous and selfless man and loving father, one who considered him a son and opened his home to Desroches when the two had only just met.

"He didn't know me from a pile of dirt on the side of the road. I just have to do right by him," he said.

"If it wasn't for him I wouldn't be becoming a father anymore."

A memorial post on Facebook, apparently written by Alysha, noted the man's propensity for kindness.

"He was truly one of a kind from his singing to his jokes he always made sure everyone around him was happy," said the post.

"We lost a true hero, I love you more then words can describe dad and I'm trying to stay strong cause I know it breaks your heart to see me cry."

House condition under investigation

A friend of Bonchek's told CBC News she wanted to move her family out of the house and complained about it needing repairs. 

The friend, Kathy Girotto, said it wasn't a "great environment" for the single mother and three kids.

"She lived for her children ... she died for her children," Girotto said. "Her children were her life until the very end."

Oshawa fire

The cause of the fire is under investigation. (CBC)

The Office of the Ontario Fire Marshal is trying to ascertain what may have caused the fire.

"Once we determine where the fire originated, then we can examine possible ignition sources," said Richard Derstroff, an OFM investigator. "But it's going to take quite some time to excavate through all the debris."

Desroches says Macdonald was the first person in the lower unit to encounter the fire, and by the time the rest of the household noticed the flames, the kitchen had been totally engulfed.

He believes shoddy renovations on the house contributed to the tragedy, noting that a previous owner had "patched things up" poorly.

"I do not think the house was up to code," Desroches said. "In my opinion it's the reason the house went up so fast."

Investigators will try to determine if the house was up to code and equipped with working smoke alarms.