The Center for Auto Safety says 303 people died after airbags failed to deploy in two GM car brands recalled last month due to faulty ignition switches.
The defect caused cars to shut off while driving, which disabled the airbags.
GM has linked the problem to at least 12 deaths.
In a statement, the company said the conclusions made by the safety group are "pure speculation."
Recently released documents show GM first became aware of the ignition problem in 2001. That's three years earlier than previously disclosed.
