An elderly couple and their daughter have died of suspected carbon monoxide poisoning at a caravan park.
The alarm was raised at the site near Camborne, Cornwall just before 1pm on Saturday after the couple's helper was unable to contact them, Devon and Cornwall Police said.
Camborne Town Council member David Biggs described the incident as an "appalling tragedy".
Firefighters were called to break into the static caravan at Tremarle Home Park, North Roskear, where they found the bodies of three people and their pet dog. The woman was in her mid-40s. The age of her parents is not yet known.
Inspector David Eldridge of Devon and Cornwall Police said: "Shortly before 1pm this afternoon we were called to a caravan park near Camborne in Cornwall, where a helper had been unable to get a reply from an elderly couple who lived in the caravan. We were able to see that there was a figure sat in a chair but they were unresponsive to knocks at the door. Cornwall Fire and Rescue were called. They forced entry into the property and found that the three occupants were all dead."
An investigation into what caused the deaths is under way, but the incident is not being treated as suspicious.
A Cornwall Fire and Rescue Service spokesman said a crew from Camborne Community Fire Station attended the scene with a hazardous materials advisor after being called at 12.56pm.
Mr Biggs said he was shocked to hear of the deaths. "Tremarle Park is a residential caravan site and has provided homes for people for many years. It's a well established facility and is very well run. It was a shock to learn that three people have died there," he said.
Stephanie Trotter, president and director of the independent charity CO-Gas Safety, said carbon monoxide was an incredibly lethal gas. "Less than 2% of CO (carbon monoxide) can kill in between one and three minutes. It cannot be sensed using human senses of smell, touch, sight or hearing," she said.
"The Department of Health estimates that about 40-50 people die of CO and about 4,000 attend Accident and Emergency every year in England and Wales alone."