Four people believed to be in their 20s were found unconscious in a car in Yamanashi Prefecture on Sunday, with investigators speculating they may have been playing out a suicide pact.
Having been alerted around 8:25 a.m. Sunday, police rushed to a vacant lot in Kamikuishiki, where they found the four.
They were immediately rushed to hospital and were said to be in critical condition.
A clay charcoal stove was found inside the car.
Police are treating the case as a possible suicide, with attempts to identify the four under way.
In a related incident, three people in their 20s were found dead in a parked car in Tsu, Mie Prefecture, on March 5.
This was again an apparent suicide pact, arranged over the Internet.
The three died from carbon monoxide poisoning caused by a portable coal stove in the car.
In February, three others killed themselves in the same manner at an apartment in Iruma, Saitama Prefecture. This group had also become acquainted over the Internet.