Four 13-year-old boys from a local junior high school died Saturday after being found unconscious in a cave in the city of Kagoshima earlier in the day, police said.
It is suspected the four were accidentally poisoned by carbon monoxide after making a bonfire near the cave entrance, the police said. A burned-out area and a lighter were found near the cave's entrance, they added.
The victims were identified as Mizuki Nakano, Kazuki Moi, Satoshi Jitozono and Koshiro Kawagoe -- all students at Takeoka Junior High School.
Nakano was lying near the entrance and the rest were found in the back of the cave at around 4:25 p.m., the police said. Rescue personnel brought them to a hospital after receiving a call for an ambulance from a neighbor near the site. The officers wore oxygen masks while extracting the victims.
The high concentration of carbon monoxide in the cave has prevented police from conducting an on-site investigation, they said.
The cave, which branches out in several directions, was used as a bomb shelter during the war. Children sometimes play in the cave, which is situated at the foot of a steep slope just outside a residential area, the police said.