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Officials are investigating Mitsubishi Motors Corp after the company said it had found employees manipulated fuel efficiency data of more than 620,000 light vehicles it manufactured.
Local media reports showed investigators from the ministry of land, infrastructure, transport and tourism entering offices of the company's assembly plant in central Japan's Nagoya on Thursday.
On Wednesday, the company apologised for what it said was intentional falsification of mileage test data that falsely boosted fuel economy by about 5% to 10%.
Trading in Mitsubishi Motors' shares halted before the close on Thursday after the shares fell more than 20%.
Mitsubishi Motors was tarnished by a massive recall cover-up of safety defects 15 years ago.
The inaccurate tests by the Tokyo-based automaker involved so-called "minicars" with tiny engines whose main attraction is generally great mileage. Mitsubishi was reporting mileage of up to 30.4 kilometres per litre.
The inaccurate tests by the Tokyo-based automaker involved 157,000 of its own-brand eK wagon and eK Space light passenger cars, and 468,000 Dayz and Dayz Roox vehicles produced for Nissan Motor Co.
All are so-called "minicars", with tiny engines, whose main attraction is generally great mileage. They were produced from March 2013.
