Officials are investigating after 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 entering offices of the company's assembly plant in central Japan's Nagoya on Thursday. On Wednesday, the company apologized for what it said was intentional falsification of mileage test data.

Trading in Mitsubishi Motors' shares halted before the close on Thursday after the shares fell nearly 20.5 percent.

Mitsubishi Motors was tarnished by a massive recall cover-up 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 said fuel economy was falsely boosted by about 5 percent or 10 percent.