A total of five campers were found dead in South Korea over the weekend, apparently due to carbon monoxide poisoning, as they fell victim to an invisible killer lurking when camping in winter, officials said Monday.

The dead bodies of two separate families were discovered at different campsites with signs of having used charcoal grills or heaters inside zipped-up tents, according to the emergency authorities.

The first group of victims consisted of a 63-year-old man, his 58-year-old wife and their 5-year-old grandson, who were camping in Yeongdong County, North Chungcheong Province, Chungbuk Fire Service Headquarter said. Their bodies were discovered by authorities around 12:30 p.m. on Sunday.

Emergency officials arrived at the scene after receiving a call from an official at the campsite, who had discovered the family unresponsive in their tent. They had arrived a day prior, authorities said.

The other victims were a couple in their 50s who were found dead in a tent in Yeoju, Gyeonggi Province at 9:55 a.m. on Friday. Emergency authorities were alerted by another camper who notified them of “unresponsive bodies in a tent next to them.”

Authorities have found no reason to suspect suicide for either case, while further investigations are underway.

The National Fire Agency has issued a warning for possible carbon monoxide poisoning as temperatures have dropped to below zero Celsius at night.

“Please refrain from using coal and wood to keep warm while inside camping cars or tents,” it said.

“The area must be regularly ventilated even when using butane gas for heating,” it added.

The number of calls related to carbon monoxide poisoning that emergency authorities received from the beginning of 2019 to the end of 2021 was 471, according to NFA data released last year. Of the 471 calls, 85.1 percent of the reports came in during winter, from October to March here. Some 26 percent of the total calls concerned campsite accidents.