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RAMROD KEY (CBSMiami) – The cause of a fire, which sparked over the weekend killing a mother and two of her children, remains unknown.

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The fire started on the back porch, according to the fire marshal. It was assumed a grill was the cause but investigators have yet to find a grill or any parts so the cause remains unknown.

The fire started just after midnight Sunday and quickly ripped through the second floor of the home in Ramrod Key.

Monroe County investigators said the fire spread so quickly, they were unable to get inside and save 45-year-old Robin Ferrer and her two children Roman, 7, and 5-year-old Hazel.

Ferrer’s 18-year-old son Jacob Shore suffered severe burns and was airlifted to Ryder Trauma Center. He’s expected to survive. Ferrer’s 19-year-old daughter Courtney Shore got out unharmed.

Family says the deceased children’s father, James Ferrer, is paralyzed with heartache.

Teresa Fuller, the paternal grandmother, said her former daughter-in-law was a hard worker and a doting mom.

“We loved her very dearly. There’s just no words to express the loss and grief that we are experiencing at this point,” Ferrer said.

The grandmother said Roman was a very bright boy.

“He had definite survivor skills,” she said. “The fire marshal told us he tried to escape the fire and was found alone in the bathroom.”

Five-year-old Hazel is remembered as precocious and outgoing.

“Both of them gave us an enormous source of energy and joy,” the grandmother said.

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Alice Bethel, the childrens’ great grandmother, cried remembering the kids.

“It’s the worst pain anybody can go through,” the great grandmother said. “I was very close to them. I would pick them up every Wednesday and take them to Burger King or McDonald’s or Dairy Queen,” said said.

Monroe County’s fire marshal, Capt. Jim Leonard, said rental homes are required to have smoke detectors, and tenants are responsible for keeping them with charged batteries.

Leonard said the upstairs of the home was too badly burned to determine if there were any smoke detectors there.  One smoke detector was found downstairs, disconnected.

The fire marshal said whether you own or rent, you should make working smoke detectors your responsibility.

“If you don’t have smoke detectors, you’re pretty much going to die, to put it bluntly,” Leonard said.  “Please have your smoke detectors operational.”

Some neighbors said they helplessly watched as the fire ripped through the home.

“The second floor up was nothing but fire so the whole roof went up. They went in and tried to save the people,” said neighbor Donna Wilson.

The father was reportedly renting a home nearby to be close to his kids.

“The poor father was on the ground. The firemen had to hold him down cause he wanted to go get them,” Wilson said.

A Go Fund Me page has been set up to raise money for Jacob and Courtney for funeral, medical and living  expenses.

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CBS4’s Gary Nelson will have more on CBS4 News starting at 5 PM.