US federal investigators are looking into whether General Motors acted quickly enough to recall 1.6 million older-model small cars in a case linked to 13 deaths.
On Tuesday, GM doubled the number of cars in a recall for faulty ignition switches.
Documents show GM knew of the problem as early as 2004.
The recall affects some older models of the Pontiac Solstice and Saturn Sky sports cars, the Saturn Ion, Chevy HHR SUV, Chevy Cobalt and Pontiac G5.
General Motors could face a fine of up to 35-million dollars for delaying a recall of several vehicles for a problem linked to 13 deaths.
G-M has apologized, saying it wasn't robust enough in examining an ignition switch problem in older-model vehicles including Chevy Cobalts, Pontiac G5s and Saturn Ions.
G-M says a heavy key ring or jarring from rough roads can cause the ignition switch to move out of position and shut off the engine and electrical power.