The preliminary investigation is expected to be opened on Thursday and involves an estimated 500,000 vehicles. The official spoke on condition of anonymity because the department had not yet notified Toyota of the probe.
Toyota Motor Corp. said at a news conference in Japan earlier Wednesday that it was looking into complaints of power steering problems with the Corolla and was considering a recall as one option. Toyota said there have been fewer than 100 complaints.
In the U.S., the National Highway Traffic Safety Administration has received about 150 complaints from drivers about the issue. The complaints are small compared to the number of vehicles involved: The department said the investigation involves about 363,000 vehicles from the 2009 model year and about 136,000 vehicles from the 2010 model year.
In the Corolla complaints, drivers have said they felt they were losing control over the car's steering, particularly at highway speeds.
Executives in Japan said it was unclear why drivers were experiencing the problems but it could be related to the braking system or tires. They said the company's internal investigation was still preliminary and no decision had been made but Toyota was prepared to supply fixes
-- including a recall as one possibility -- if it found defects.
Toyota has recalled 8.5 million of its vehicles, including the popular Camry
-- America's best-selling model -- over problems with gas pedals, floor mats and brakes.
The 2009 and 2010 Corollas are already the subject of two separate safety-related recalls, one for potentially sticky accelerator pedals, the other over concerns that gas pedals could get caught on floor mats.
A Toyota spokeswoman in Washington, Cindy Knight, said the company would cooperate with the investigation.