A Waltham Police officer who was about to turn around as he neared the Newton line was in the right place at the right time to likely save the lives of five residents unknowingly sleeping through a house fire.

“They’re all lucky to be alive,” said Heather MacDonald, whose mother Jean and sister Megan lived in the house that was gutted by the blaze on May 29. “If he hadn’t seen it, they all probably wouldn’t have made it out.”

Officer Tony Scicholne was on Derby Street at about 5:30 a.m., when he spotted a porch fire. He called in dispatch, then contacted the Newton Fire Department.

While waiting, Scicholne got all the residents out just before one side of the house erupted into flames. Megan MacDonald wasn’t home at the time, but Heather MacDonald said her mother, a heavy sleeper, was woken by the pounding on her door.

“She called me and said ‘My house is on fire!’” recalled MacDonald, a Newton resident. “I said ‘Get out! I’m on my way!’”

Jean MacDonald made it out with her purse, coat, pajamas and nothing else, Heather said. Firefighters managed to rescue two cats from the second floor, but the top floor, where Jean lived, was totally burned.

The fire spread quickly, Heather MacDonald said.

“By the time my mother got to the corner, the whole side of the house was engulfed, but when she first looked at it, it was just the porch,” MacDonald said. “She turned around and the whole side was up, it just blew through the entire house.”

The fire quickly went to three alarms, with Waltham sending engines and Brookline, Needham, Watertown and Wellesley providing station coverage, according to Newton Fire Chief Joseph LaCroix. No firefighters were injured in the blaze.

LaCroix said several propane tanks and two five-gallon jugs of gasoline were under the porch when the fire started. While the tanks did not explode, a few had loose valves and the flammable gas fed the fire, he said.

Fire department investigators are still determining the fire’s cause, but LaCroix said they were leaning toward careless disposal of a cigarette.

MacDonald said her mother and sister are living with her for the time being. She praised Mayor Setti Warren’s office for assisting the victims and landlord/owner Fred Camerato for going “above and beyond.” The house was Camerato’s childhood home, MacDonald said, so the fire is a personal loss for him as well.

But MacDonald was most concerned for her mother.

“She’s going to walk out of this apartment with nothing, you lose everything in a fire like this,” MacDonald said. “Just watching all her stuff go up … it’s very sad.”