Plaistow, NH – Rescue crews forced their way inside a local home Tuesday to find two men and one woman lying dead in their beds still in their pajamas and a fourth man unconscious — all apparent victims of carbon monoxide poisoning.

Firefighters were called to the 5 Center Circle Cape-style home for a welfare check about 11:30 a.m. after friends of the four occupants told police they were unable to contact them for the last few days, Plaistow Fire Chief John H. McArdle said.

The four victims — who all appeared to be at least in their 30s — were not related to each other, but apparently were work associates who roomed together, McArdle added.

The batteries had been removed from their carbon monoxide detector, he said.

The victims’ names were not available because all their identities were not immediately known and family members had to be notified, McArdle said.

Town records list the owner of the house as Kirk Walsh.

When firefighters got inside the home, they found two adult men and one adult woman dead, McArdle said. A fourth man still had a pulse. He was taken by ambulance to Lawrence General Hospital in Lawrence, Mass., and, from there, transported by ambulance to Massachusetts General Hospital in Boston.

The carbon monoxide level in the home was more than 300 parts per million, McArdle said. That is about 10 times higher than what would set off a normal carbon monoxide detector, he said.

Cause of the poisoning appears to be a problem with the home’s propane-fired heating system, which was located in the basement.

The New Hampshire Fire Marshal’s Office is investigating with the help of Plaistow police and fire departments.

McArdle said it’s important for all homeowners to make sure they have working smoke and carbon monoxide detectors and that heating systems are checked regularly by qualified technicians.

“It is important to ensure all vents from your heating system are kept clear of snow and ice, particularly with the amount of snowfall we have had this winter,” McArdle said.

The potential for carbon monoxide poisoning is a major concern for the state fire marshal’s office and local firefighters.

In January, a Manchester family at 243 Auburn St. was gathered around a gas heater in a parlor. Seven were overcome by carbon monoxide fumes; all recovered.

Fire officials determined that the heater had malfunctioned and was the source of the deadly fumes.

In March 2013, a Barrington couple died of carbon-monoxide poisoning. Officials said David and Barbara Miller, both 61, were found dead in their home after they were exposed to a high level of the gas— about 500 parts per million — which probably came from their oven.

Just last week, state Fire Marshal J. William Degnan issued a warning to state residents that urgent steps are needed to reduce fire and carbon monoxide deaths during extreme winter weather.

So far this year, there have been five fatalities in the state from fires in residential homes. The Plaistow deaths would be the first this year from carbon monoxide, according to Degnan.

“It is the deadliest time of the year for home fires and carbon monoxide poisonings,” Degnan said. “As families struggle to stay warm, it’s important to clear flues and outside exhaust vents of snow and debris, place generators outside if there is a power outage, and keep flammable materials at least three feet away from any heat source. These small but vital actions can be life-saving.”