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.
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.
Andrew Harding of the BBC 'Newsnight' program examines how the Internet and suicide have come together in Japan with the most tragic consequences, with more than 30 deaths since October. Seven strangers who'd just met on-line in a suicide chat room drove out of Tokyo and up into the mountains.
The suicides were organised by two women who met over the Internet and tried but failed in a previous attempt to kill themselves together.
KERRY O'BRIEN: Suicide seems a permanent and tragic blot on the modern social landscape.
But try wrapping your head around the concept of a group cyber suicide pact involving people who are strangers to each other.
ANDREW HARDING: And there are thousands more apparently sincere messages about pills and despair and death.
Japan has always had a quiet fascination with suicide.
They were still trying to determine the identities of the men and women.
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.