Mitsubishi Motors Corp. admitted Tuesday that it has falsified fuel economy tests since 1991. The latest revelation comes after the Japanese automaker said last week that it had altered mileage tests on about 625,000 light vehicles.

“For the domestic market, we have been using that method since 1991,” Mitsubishi Vice President Ryugo Nakao said, according to Agence France-Presse. “But we don’t know the number of models affected,” he added.

Also on Tuesday, Mitsubishi announced in a statement that it has set up a three-member investigation committee to look into the fuel test scandal. The committee will investigate the matter for about three months following which it will form a report, the statement read, adding that Mitsubishi will make the report public in a “timely manner.”

“[Mitsubishi] expresses its most sincere apologies to all of our customers, shareholders, and stakeholders for any inconvenience or concern caused by this occasion,” the company statement said.

Mitsubishi has reportedly lost about $3.9 billion of its market value since the scandal came to light last week. This is the first time a Japanese automaker has been involved in manipulating fuel-economy tests. German carmaker Volkswagen was last year found to have cheated in diesel emissions tests in the United States and other countries.

Last Thursday, Japanese Transport Ministry officials raided a Mitsubishi office over the matter. The Japanese government is likely to impose a fine and has asked the country's sixth-largest automaker to submit a full report over the altered data by Wednesday.