Millikin also said the warnings weren't reported to the U.S. Securities and Exchange Commission until after the recalls began earlier this year.
McCaskill, a former prosecutor, said she can't understand why Millikin and one of his top deputies still are with GM.
"This is a either gross negligence or gross incompetence on the part of a lawyer," she said. "I think the failure of this legal department is stunning."
Millikin said he only learned about the ignition switch problems in February and acted quickly once he did.
Senators also focused on how GM failed to answer requests for information from the National Highway Traffic Safety Administration on fatal crashes. GM responded to the so-called "death inquiries" by asserting attorney-client privilege or saying it had not assessed the cause of a particular crash, said Sen. Barbara Boxer, D-Calif.
"I consider it a cover-up when a manufacturer does not respond fully and accurately to NHTSA," Boxer told Millikin.
Boxer asked Barra if the people who gave "non-answers" to safety regulators had been fired. Barra said she believed they had.
Those regulators will soon have to give Congress answers of their own.
Senators say the safety agency missed signs of Chevrolet Cobalt stalling problems and failed to hold GM accountable. Sen. Edward Markey, D-Mass., said the agency ignored consumer complaints and written reports from its own contractors that linked the switches to air bag failures in two fatal crashes.
"I think the whole story has to get out there," Markey said.
Durbin reported from Detroit. AP Auto Writer Tom Krisher also contributed to this report.