Mr. DeGiorgio not only approved the switch, the report said, but after modifying it in 2006 by making it harder to turn, he helped hide the problem when he failed to change the corresponding part number.

But Mr. DeGiorgio told congressional investigators that he had not acted alone, and that other G.M. engineers had been involved in the approval process because the switch had other problems.

Mr. Valukas’s report also said that Mr. DeGiorgio was responsible for fixing the switch, and signed off on the change. But congressional staff members say that at least five G.M. employees appear to have been aware of the change. And a G.M. “supply quality engineer” signed off on it with Mr. DeGiorgio, according to the investigators.

Beyond Mr. DeGiorgio’s role, the panel wants to ask Ms. Barra whether cost and time pressures at G.M. influenced Mr. DeGiorgio to approve the switch, even though it did not meet G.M. specifications for the amount of force needed to turn the key. At the time, early in the last decade, G.M. was reeling from billions in losses. It would eventually tumble into bankruptcy in 2009.

“The series of questionable actions and inactions uncovered in the investigation were inexcusable,” Ms. Barra wrote in her prepared remarks, and she plans to tell lawmakers she had already begun a program that encourages employees to report safety problems.