Since February, G.M. has recalled 2.6 million Chevrolet Cobalts and other small cars to fix the defect, which the company so far links to 13 deaths and 54 accidents.

But Ms. Barra admitted that the tally of deaths, injuries and crashes could grow in the weeks ahead, as the compensation expert Kenneth R. Feinberg prepares a report on how G.M. will make restitution to accident victims and their families.

Ms. Barra said the company had dismissed 15 employees as a result of the report, and disciplined five others — highly unusual in an industry where such purges have been rare. She declined to provide details about the actions, but said that more than half of those who left the company held senior-level positions.

Among them were Michael J. Robinson, a vice president for global regulatory affairs, and William Kemp, a top lawyer who oversaw product-related litigation, according to a person briefed on the moves.

Those dismissed also included two safety executives, Gay Kent and Carmen Benavides, as well as two midlevel engineers, Raymond DeGiorgio and Gary Altman, both of whom had previously been suspended for neglecting to address the switch problem in its early stages.

“I never want to put this behind us,” Ms. Barra told employees. “I want to keep this painful experience in our collective memories.”