Lawmakers are expressing skepticism about some findings of a General Motors' investigation into its mishandled recall of 2.6 million small cars as CEO Mary Barra appears before Congress Wednesday.

According to a congressional aide, members have more questions about how much Barra knew about a problem with the ignition switches in the cars when she was GM product development chief. Barra said in previous testimony in April that she first learned of the problem late last year. The GM report exonerated Barra and other top executives.

The aide requested anonymity because the questions haven't been made public.

Former U.S. Attorney Anton Valukas, who was hired by GM to do the investigation, will also testify. Panel members will question Valukas on his conclusions that a lone engineer, Ray DeGiorgio, was able to approve the use of a switch that didn't meet company specifications, and years later, to order a change to that switch without anyone else at GM being aware, the congressional aide said.

The panel's own investigation turned up evidence that at least five other GM employees were aware that DeGiorgio ordered the company that made the part to change the switch, the aide said.

Mary Barra

Mary Barra, CEO of General Motors, testifies before Congress today. (CBC)

Barra plans to tell lawmakers that she's taken steps to fix GM in the wake of the small car recall. She authorized a companywide safety review that has led to more than 40 recalls this year that cover almost 18 million cars in the U.S. GM has said that more are possible.