Three men were found dead inside a tent in a forest in Isumi City, Chiba Prefecture, on Monday morning.

Police said the men had gone into the mountains on Sunday to go duck hunting, Sankei Shimbun reported. An acquaintance, looking for them, discovered the trio collapsed inside their tent around 8:30 a.m. on Monday.

The three men, two in their 70s and one in his 60s, were confirmed dead at the scene by firefighters. According to the fire department, the group appeared to have died in their sleep.

The men had no visible injuries, but a kerosene heater and lantern were among the equipment discovered. Investigators suspect that the three may have died from carbon monoxide poisoning.