At least one senior officer, the general counsel Michael P. Millikin, whose department has been a major focus of the inquiry, has been assisting Mr. Valukas.
Company officials, who spoke on condition of anonymity, said Ms. Barra, who has been briefed on the investigation's progress, cleared a critical hurdle last month when the nation's auto safety agency imposed a $35 million fine on G.M. for failing to report the defect in a timely manner.
David J. Friedman, the acting chief of that agency, the National Highway Traffic Safety Administration, was asked on May 16 if his review of 200,000 internal G.M. documents showed that Ms. Barra knew of the problem before the recall was ordered.
"I don't have any records of that," he said.
Read MoreThe corporate culture: Behind the scenes at General Motors
Since then, G.M. officials have been more aggressive in defending Ms. Barra, both publicly and privately. One official said that evidence linking her to the delay did not exist. And the company's previous chief executive, Daniel F. Akerson, broke his silence on the subject in an article about Ms. Barra in Forbes magazine.
"She didn't know about it," Mr. Akerson was quoted as saying. "I bet my life on it."
In recent weeks, Ms. Barra has subtly stepped up her traditional duties as chief executive, including holding more employee meetings, interviewing high-level job candidates, and presiding at the start of the G.M.-sponsored Grand Prix race in Detroit last Sunday.