The switches, which can slip out of the "run" position and cut off the engine, have been linked to at least 169 deaths.
Despite evidence that GM's legal and engineering staffs concealed the problem for nearly a decade, no employees were charged, though US Attorney Preet Bharara said the investigation is still going on.
Also today, GM announced it will spend USD 575 million to settle the majority of the civil lawsuits filed over the scandal.
Two charges drawn up against GM wire fraud and scheming to conceal information from government regulators will be dropped after three years if the automaker cooperates fully.
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GM agreed to a statement of facts that describes in scathing terms its deceptive and dismissive approach to the defect.
"They let the public down," Bharara said. "They didn't tell the truth in the best way that they should have to the regulators, to the public about this serious safety issue that risked life and limb."
With the settlements, GM takes a big step toward moving past the scandal.
GM chief executive Mary Barra appeared before several hundred employees in suburban Detroit and again apologized to the crash victims. When asked whether employees should have been charged, she deferred to Bharara's office.
But she added: "We understand that lives were impacted. That is something that we understand and we take forward and will have with us every day."