Mitsubishi Motors Corporation, the Japanese car maker, which last week admitted manipulating emissions data in more than 600,000 vehicles, said an internal investigation found such tampering dated to 1991.
Company president Tetsuro Aikawa told reporters on Tuesday that the probe was ongoing, suggesting that more irregularities might be found.
"We don't know the whole picture and we are in the process of trying to determine that," he said at a news conference at the transport ministry in Tokyo. "I feel a great responsibility."
Aikawa said so much was unknown that it was uncertain what action the company would take. He said he didn't know why employees resorted to such tactics to make mileage look better.
Mitsubishi had repeatedly promised to come clean after a massive scandal 15 years ago involving a systematic cover-up of auto defects.
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 Motor Company.
The models are all so-called "minicars" with tiny engines whose main attraction is generally great mileage. They were produced from March 2013. The problem surfaced after Nissan pointed out inconsistencies in data.
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Aikawa also said it was unclear how customers were going to be compensated because the extent of the cheating was still under investigation.
Mileage fraud is a violation of Japan's fuel efficiency law for autos because buyers are eligible for tax breaks if a vehicle model delivers good mileage. Possible penalties are still unclear because of the uncertainties over the investigation's outcome, according to the transport ministry.
Mitsubishi released two diagrams explaining how road tests were manipulated.
The company, which also makes the Outlander sports utility vehicle and the i-MiEV electric car, has arranged for a panel of three lawyers, including a former prosecutor, to further investigate the mileage scandal from an outsider's point of view. A report is expected within three months.
Production and sales of all affected models have been halted.
Japan is periodically shaken by scandals at top-name companies, including electronics company Toshiba Corporation, which had doctored accounting books for years, and medical equipment company Olympus Corporation, which acknowledged it had covered up massive losses.
Mitsubishi struggled for years to win back consumer trust after an auto defects scandal in the early 2000s over cover-ups of problems such as failing brakes, faulty clutches and fuel tanks prone to falling off dating back to the 1970s. That resulted in more than a million vehicles being recalled retroactively.