Firefighters say a family is lucky to be alive after their unit in a Cambridge duplex was destroyed by fire early Monday morning.

People living near 125 Park Avenue said they heard a loud bang coming from the house around 12:20 a.m.

An hour later, they saw flames shooting out of the residence’s front door, called 911 and ran over to help.

The family of four inside was able to get to safety.

Firefighters were able to make quick work of the fire, but by that point everything inside the building was destroyed.

“As much as we’re happy that they’re all OK … they’ve lost everything,” said Nancy Schneider, whose daughter, son-in-law and two grandchildren were inside.

“It’s just heart-wrenching.”

Firefighters say the fire began with unattended candles.

The family living inside may have been alerted to it earlier, they say, had there been batteries in all of their smoke alarms.

“Smoke alarms are important. They should be on every level of your home, and outside all sleeping areas,” said Dana Landsborough of the Cambridge Fire Department.

The family didn’t have insurance coverage for the contents of their home.

Danielle Taylor doesn’t know the family inside, but their loss – especially coming less than a month before Christmas – spurred her to call for donations for the family via Facebook.

“People, once they hear the story, are more than happy to help,”

Furniture, clothes, toys and cribs are among the items being collected.

Damage from the fire is estimated at somewhere between $100,000 and $150,000.