The U.S. government's auto safety watchdog says it will investigate whether General Motors acted quickly enough to recall 1.6 million older-model small cars.
The National Highway Traffic Safety Administration says it will investigate if GM properly followed legal requirements to report safety problems to the agency.
The agency can fine GM as much as $35 million.
GM on Tuesday doubled the number of cars in the recall for faulty ignition switches. The problem has been linked to crashes causing 13 deaths.