(Reuters) - U.S. safety regulators have opened an investigation into whether General Motors Co. reacted fast enough in its recall of more than 1.6 million cars over an ignition-switch defect linked to 13 deaths in crashes.

"The National Highway Traffic Safety Administration has opened an investigation into the timeliness of General Motors' recall of faulty ignition switches to determine whether GM properly followed the legal processes and requirements for reporting recalls," the safety agency said in a statement released Wednesday.

GM could face a maximum fine of $35 million if it failed to notify NHTSA within five days of a recall after learning of a vehicle safety defect.

The recall was to correct a condition that may allow the engine and other components including front airbags to be unintentionally turned off.

The Detroit company previously said the weight on the key ring, road conditions or some other jarring event may cause the ignition switch to move out of the "run" position, turning off the engine and most of the car's electrical components.

NHTSA urged owners to follow GM's recommendation to "use only the ignition key with nothing else on the key ring" when operating the vehicle and seek the repair as soon as replacement parts become available. NHTSA said it will monitor the recall and take additional action as needed.

On Tuesday GM more than doubled its recall related to the issue, saying it was "deeply sorry" and that the company was reviewing its recall process, acknowledging it was not as "robust as it should have been. GM said then that it was aware of 31 reported incidents, including 13 front-seat fatalities, involving frontal crashes in which the condition may have caused or contributed to the front airbags not deploying.

Earlier this month, GM said it was recalling 778,562 Chevrolet Cobalt and Pontiac G5 compact cars from model years 2005 through 2007. On Tuesday, it added 842,103 Saturn Ion compact cars from 2003 through 2007 model years, Chevy HHR mid-sized vehicles from 2006 and 2007, and the Pontiac Solstice and Saturn Sky sports cars from 2006 and 2007.

GM no longer makes any of the affected cars. It previously said it is working with suppliers to increase production of replacement parts and accelerate the process. Dealers will replace the ignition switch at no charge.

GM said the ignition switch torque performance may not meet company specifications. The involved parts were made in Mexico, according to documents previously filed with the U.S. National Highway Traffic Safety Administration.

Of the cars recalled, 1,367,146 vehicles are in the United States, 235,855 are in Canada, 15,073 are in Mexico and 2,591 were exported outside North America, according to GM.