US safety regulators have recorded 303 deaths when airbags failed to deploy in 1.6 million compact cars recalled last month by General Motors, according to a study released by a safety watchdog.
The new report and higher death toll ratchet up the pressure on GM, which has said it has reports of 12 deaths in 34 crashes in the recalled cars.
GM did not recall the cars until February, despite learning of problems with the ignition switch in 2001 and issuing related service bulletins to dealers with suggested remedies in 2005.
The carmaker is facing increasing pressure to compensate victims and create a US$1 billion fund, even if some would-be plaintiffs are barred from suing under the terms of GM's emergence from bankruptcy in 2009.
The Centre for Auto Safety said it referenced crash and fatality data from the US National Highway Traffic Safety Administration's (NHTSA) Fatal Analysis Reporting System (FARS).
GM said the new report was based on "raw data" and "without rigorous analysis, it is pure speculation to attempt to draw any meaningful conclusions".
Clarence Ditlow, the centre's executive director, said, "NHTSA could and should have initiated a defect investigation to determine why airbags were not deploying in Cobalts and Ions in increasing numbers."
GM recalled the cars because when the ignition switch is jostled, a key could turn off the car's engine and disable airbags, sometimes while travelling at high speed.
The safety agency has been criticised for not pressing GM to recall the cars with defective switches, despite receiving hundreds of consumer complaints in the past 10 years and implementing its own investigations of two deaths related to the faulty ignition switches.
US Transportation Secretary Anthony Foxx promised an "aggressive investigation" into whether GM was slow to report to the federal government problems with ignition switches on the 2005-2007 Chevrolet Cobalt and 2003-2007 Saturn Ion.
The US attorney in Manhattan has opened a criminal probe, and House and Senate committees have pledged hearings about GM and NHTSA's behaviour.
Ditlow said the centre's study also cross-referenced fatality data supplied by GM to NHTSA's Early Warning Reporting (EWR) database.
"Combining EWR and FARS data as [the centre] did should have raised a red flag to NHTSA," Ditlow said in a letter sent to the safety agency.
GM said its investigation into the recall and the impact of the defective switch was "ongoing".
GM's slow recall, 13 years after the company first saw signs of a problem, is the subject of several investigations.