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Three people - a 21-year-old university student, a 25-year-old man and a 20-year-old woman with a part-time job - were found dead in a car in the southern city of Fukuoka on Sunday, police said.
The suicide pact was suspected to have been arranged over the Internet as the three came from different parts of Japan.
Investigators found four charcoal stoves in the car that they believe the group used to poison themselves.
No external wounds or signs of a struggle were found.
The Saitama police said they believed the seven died of carbon monoxide poisoning, and ordered autopsies.
In a separate incident, two women were found dead in a car parked outside an isolated temple in Yokosuka, about 60 miles to the southeast, a Kanagawa prefecture police spokesman said.
More recently, Japan's economic slump has driven many struggling businessmen to take the honourable way out, like the three men found in this car. Now it's younger, lonelier voices calling up the help lines and driving up the suicide statistics, even as Japan's economy is on the mend.
The women answering these phones night after night blame, in part, the Internet.
But try wrapping your head around the concept of a group cyber suicide pact involving people who are strangers to each other.
It's become something of a craze in Japan, where the act of ritual suicide, including that of wartime Kamikaze attacks, has always had a cultural significance.
In October, nine people were found dead from carbon monoxide poisoning in rented vehicles near Tokyo.
The suicides were organised by two women who met over the Internet and tried but failed in a previous attempt to kill themselves together.
They have claimed dozens of lives and shocked Japan over the past several years.