An internal investigation by Mitsubishi Motors has found the company has been meddling with fuel economy data since 1991.
Company President Tetsuro Aikawa said: "Although the investigation has progressed to a certain point, the reason and those who were responsible remain unclear, so the investigation will continue."
"We don't know the whole picture and we are in the process of trying to determine that," he said at a news conference. "I feel a great responsibility."
Japan's sixth-largest auto maker has lost half of its market value -- some US$3.9 billion -- since it admitted last week that it overstated the fuel economy of four domestic mini vehicle models, including two produced for Nissan Motors.
Asked how the company plans to stay afloat, Mr Aikawa replied: "Until we get an idea of when this investigation will end, I can't answer your question. That's how big of a problem I feel this is."
The inaccurate mileage tests involved 157,000 of its eK wagon and eK Space light passenger cars, and 468,000 Dayz and Dayz Roox vehicles produced for Nissan.
The revelations have sparked fears of ballooning compensation costs and fines, and prompted Japanese authorities to raid one of its research and development facilities.
The Japanese transport ministry has set up a task force to examine how other automakers submit fuel economy data.