Sashalynn Rosa and her 1-year-old son Messiah died, and a 3-year-old daughter was in critical condition, police said

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A New Jersey mother and her infant son were killed Saturday, and her toddler daughter was in critical condition, after their car filled with carbon monoxide – due to a tailpipe clogged with snow – as the father was digging the vehicle out from the weekend storm, authorities said.

The fatal victims were Sashalynn Rosa, 23, and her son Messiah, 1. Daughter Saniyah, 3, was in critical condition as of Sunday night, Passaic detective Andrew White told

“It’s hard to lose them like that,” the boy’s grandfather, Felix Bonilla, told WABC-TV. Bonilla also claimed the prognosis for Saniyah was not good. But on Monday, her condition began to improve, according to Passaic mayor Alex Blanco.

“Doctors are saying the girl is showing some improvement,” Blanco tells PEOPLE. “She is breathing on her own now.”

The family went outside to play in the snow and had gone into the car to warm up while the woman’s boyfriend was shoveling around it, Blanco says.

“What they didn’t realize is that the snow is so high it covered the tailpipe,” he says. “It didn’t register that it is a cause of carbon monoxide poisoning. We’re aware of carbon monoxide in a closed garage, or at home with gas heaters, but this doesn’t register.”

“It was such an unfortunate accident,” he continues. “The boyfriend was happily cleaning the car and the family was in the car warming up and he noticed they fell asleep, they were not awake.

“He knocked on the window, they didn’t wake up. He opened the doors to try and wake them up, he couldn’t. He started yelling and screaming and people came in to get them out and call 911.

“Some of the residents tried to give them CPR before the ambulance came – it came and they tried to resuscitate them. They got a weak pulse from what I heard, took them to the hospital and the mother died and the 1-year-old died. The 4-year-old is fighting for her life in a coma as we speak at St. Joseph’s. The father is there and most of the family.”

“It happened quick – in 10 or 15 minutes – and it doesn’t smell,” he adds. “There was no smoke inside the car that would have been a red flag for them.”

On Twitter, Blanco called the deaths “a scar on our city.”

“From what I heard, they are simple people, hard-working people,” Blaco tells PEOPLE. “They didn’t own a house, they lived in an apartment – just a regular young couple with two kids, trying to make a living.”

A GoFundMe page was created to raise money for funeral costs.

The risk of carbon monoxide poisoning due to a snow-clogged tailpipe is not often mentioned when winter storms hit, Mayor Blanco acknowledged. “I probably would have made the same mistake,” he admitted.

But recognizing that risk is among the winter safety tips outlined by numerous organizations, including the National Highway Traffic Satefy Administration.

The NHTSA says: “To avoid carbon monoxide poisoning when stuck in snow, be sure to keep your vehicle’s exhaust pipe clear of snow and ice, run your vehicle only in the open with the windows partially down, and run it only long enough to keep warm.”

Reporting by DIANE HERBST