Daihatsu, a subsidiary of the automobile giant Toyota, announced that it was forced to shut down its production lines at all four of its Japanese factories December 26 while the company is being investigated for improper safety certification tests by transport ministry officials.

According to the Associated Press, the shutdown comes about a week after Daihatsu announced it was suspending all vehicle shipments in and out of the country after it was found that the organization used improper testing for 64 vehicle models. The suspension caused transport ministry officials to launch a larger investigation into testing problems that have allegedly gone on for decades.

The issues began in April 2023 when Daihatsu reported improper testing on the door linings of cars. A month later, side collision testing issues surfaced, officials said. Data falsification and unauthorized testing procedures were also discovered within the company.

The safety test issues triggered an independent investigation earlier in 2023, which found widespread and systemic problems within the company. So far, there have been no reports of deaths or accidents linked to the falsified tests.

Daihatsu said it began shutting down some assembly lines December 25 before stopping production at all four of its plants in Shiga, Kyoto, Oita and Osaka just one day later. Though the company did not reveal when it will begin production again, some media outlets said that the suspension could last through at least January.

Daihatsu is a unit of Toyota that specializes in manufacturing small cars and trucks that are popular in Japan. During the 2022 fiscal year, the organization assembled roughly 870,000 vehicles at its four Japanese plants.