Investigators want to know how long Toyota knew about acceleration and brake problems before they issued recalls.
The investigation could help authorities determine whether Toyota violated federal law by issuing its recalls too slowly.
The law requires an automaker to promptly issue a recall and notify federal regulators within five days of finding a safety defect.
If found in violation, Toyota could face up to $16 million in fines.
Investigators are also reviewing how closely Toyota looked at whether electronics played a role in the acceleration problem.
Complaints to the federal government blame as many as 34 deaths on unintended sudden acceleration problems in Toyotas.