Shares of Mitsubishi Motors plummeted for a fifth straight day as a result of implications that the company has improperly tested the fuel efficiency of its vehicles from as far back as 1991. The decline has reduced the market value of the Japanese manufacturer by 50 percent, to 427 billion yen ($3.8 billion).

Mitsubishi has appointed a panel comprised of three former prosecutors to investigate the scandal over a period of the next three months. The investigation will determine the number of affected vehicles and provide details of a compensation plan.

Mitsubishi is also in discussions with partner Nissan about how it will reimburse the company. Nissan was supplied with about 75 percent of the 625,000 affected minicars that it resold to customers in Japan. Nissan has issued stop sales on the Dayz and Dayz Rooz as a result of the scandal.

The U.S Environmental Protection Agency and the California Air Resources Board have also initiated an investigation to determine if Mitsubishi models sold in the U.S. have complied with domestic fuel economy regulations.