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Investigators said they haven't yet figured out what caused the apparent leak of carbon monoxide from the propane gas heating system.

New Hampshire Fire Marshal Sean Toomey said there were no carbon monoxide alarms found in the home, and he urged people to install and maintain such alarms.

“This has been a terribly tragic year for both fire deaths and carbon monoxide deaths,� Toomey said. “To lose a whole family of four right in the peak of the holiday season hits home even more."

The Centers for Disease Control and Prevention describes carbon monoxide as a colorless, odorless gas that can kill without warning. The CDC said more than 400 Americans die each year from unintentional carbon monoxide poisoning not linked to fires, while another 14,000 are hospitalized.

Police arrived at the house in the town of Wakefield about 4:20 p.m. on Christmas Day after the family didn't show up as expected at an extended family gathering. Other relatives had tried messaging and calling each of the four, Toomey said. Those relatives then called police, who conducted a welfare check, he said.

Toomey said police found three of the victims in bedrooms and the fourth in a bathroom, and that authorities suspect they died during the night. When fire crews arrived, they found the home had elevated levels of carbon monoxide, Toomey said, adding that the deaths were believed to be accidental.

New Hampshire’s Office of the Chief Medical Examiner completed an autopsy of Matthew Goldstein and found his cause of death was carbon monoxide poisoning. Autopsies have yet to be completed on the other three victims.

Matthew Goldstein taught eighth grade at the Edith C. Baker School near Boston, which serves the Chestnut Hill and South Brookline neighborhoods of Brookline.

“Mr. Goldstein’s dedication to inspiring students and shaping young minds has left an indelible mark on all who had the privilege of knowing him. Our thoughts and prayers are with his family, friends, colleagues and students during this profoundly difficult time,� Brookline Public Schools Superintendent Linus Guillory said in a statement.

Matthew Goldstein had also performed in many plays for the Newton Theatre Company.

Lyla Goldstein's LinkedIn profile indicated she had worked at Microsoft for more than 16 years and for the last four years had been a principal project manager, focusing on projects that involved data and artificial intelligence.

Valerie Goldstein's LinkedIn profile said she was working as a fifth grade teacher in eastern North Carolina, in the first year of a Teach for America commitment after graduating from Syracuse University. She said in her profile that she hoped for a future career in education policy and advocacy.

Violet Goldstein was a freshman at the Rhode Island School of Design, school President Crystal Williams said.

“I know that you join me in feeling utter heartbreak at the loss of such a promising young member of our community, along with her family, on what should have been a joyous occasion,� Williams wrote in an email to the school's community.

Tax records indicate the home, on the northern tip of Balch Lake, was owned by the couple. A listing on Redfin says the home, which includes a pool and a dock, was newly renovated and had a new, efficient heating system.

Temperatures in Wakefield on Christmas Day reached a low of about 13 degrees Fahrenheit (-11 degrees Celsius).

Toomey said investigators believe the heating system in the home malfunctioned, allowing carbon monoxide to escape outside of the normal venting system. He said leaks can happen whenever there isn't efficient combustion in propane, oil or wood heating systems.

As well as having alarms, he said, people should ensure their chimneys are routinely cleaned and their systems serviced. They should also check during winter that vents aren't blocked by snow or ice, he said.