GM has said it knows of 13 fatalities in more than 30 accidents which arose from the ignitions, which could shut off a car's electrical systems including airbag deployment while the car was operating.
The automaker has recalled 2.6 million cars from model years 2003-1011 and was fined a record USD 35 million by the National Highway Traffic Safety Administration (NHTSA) earlier this month for hiding the problem for years.
GM also faces lawsuits by victims of the accidents which analysts say could eventually tote up to billions of dollars.
"They deserve straight answers about what happened to their loved ones," the agency said.
Also Read
"The final death toll associated with this safety defect is not known to NHTSA, but we believe it's likely that more than 13 lives were lost."
But it said GM is in a better position to know how many more deaths were involved, based on its information from incident reports and lawsuits.
The independent Center for Auto Safety said in a March letter to NHTSA that records show 303 deaths in accidents in which the airbags did not deploy in Chevrolet Cobalts and Saturn Ions, two of the GM models recalled over the ignition problem.
The center added that the toll was likely higher if other models also covered by GM's ignition recall are included. It criticized the NHTSA for not having earlier investigated the problem.
"NHTSA could and should have initiated a defect investigation to determine why airbags were not deploying in Cobalts and Ions in increasing numbers," it said in the March letter.