Editor's note: An earlier version of this story misspelled the first name of attorney Anton Valukas, GM's special investigator.
WASHINGTON (Reuters) -- Congressional investigators are examining whether General Motors CEO Mary Barra and other senior executives were more involved than they have publicly acknowledged as the automaker considered how to deal with a deadly ignition switch issue linked to at least 13 deaths, three sources familiar with the probe told Reuters.
The investigators also are examining whether executives acted fast enough, once they learned of the problem, said the congressional sources, who were not authorized to speak publicly about the matter.
At issue is who knew about and was responsible for an internal investigation in 2011-2013 that eventually discovered the switch issue which has led to the recall of 2.6 million cars.
GM documents, including emails and internal reports released by congressional investigators, show two former Barra lieutenants, Terry Woychowski and Jim Federico, worked with the GM field engineer who ran the internal probe from August 2011 to December 2013.
Documents reviewed by congressional investigators give no indication the two men informed Barra of the probe. But investigators are still struggling to understand GM's complex structure, who made decisions on the probe, and who was accountable for the work.
The congressional committees plan to hold hearings to question attorney Anton Valukas, who is running a current company investigation into how it handled the switch, and former CEO Dan Akerson, who ran the company during the 2011-2013 probe. Spokespeople for Akerson and Valukas said they declined to comment.
GM spokesman Jim Cain reiterated the company position that top executives did not learn of the defect until January 31 of this year, when the decision to recall was made.
"The whole purpose of the Valukas investigation is to understand the entire chronology of events and who knew what when, and what decisions were made or should have been made," he said.
Woychowski and Federico did not respond to requests for comment from Reuters.
Cain said the move had no link to the switch issue.
So far, GM has submitted about 350,000 documents to committees studying the timing of decisions, but GM has yet to produce an organizational chart that would help pinpoint lines of authority, a congressional source said. GM's Cain said he was not aware of the automaker having submitted such a chart.
GM has said that senior executives including Barra were kept out of recall investigations to allow engineers to make the decisions without undue pressure. Congressional investigators, evaluating that claim found and released one document showing a briefing on a separate safety issue, indicating at least one exception.
Woychowski emailed Barra in October 2011 and attached a copy of a New York Times report on a power steering issue in cars including the Saturn Ion and the Chevrolet Cobalt. Woychowski told Barra why the Ion had not been included in a recall of the Cobalt on the steering issue. GM spokesman Cain said that the email was part of an exchange spurred by the article.
The ignition switch was a different investigation, and Barra in April told lawmakers that she "knew there was an issue with the Cobalt" in December 2013, but she "did not know it was an ignition switch issue."
Barra in early 2011 was named senior vice president for global development, a unit which included responsibility for safety investigations and recalls, GM's Cain said. As manager of more than 30,000 employees, she supervised the "design, engineering, program management and quality of vehicles for the company's 11 brands around the world," said a GM press release.
The ignition switch problem had been a lingering issue, kicked around by GM engineers since 2001. The switch had been installed in more than 2.5 million vehicles across numerous product lines, including the Chevy Cobalt, the Saturn Ion, the Pontiac G5 and the Chevrolet HHR.
GM engineers knew the switch malfunctioned under the weight of heavy key rings. The company sent notices to dealers suggesting some drivers lighten key rings. It rejected changing the switch.
In mid-2011, for reasons that remain unclear, GM's legal and engineering departments consulted and decided to launch a formal internal investigation into airbag malfunctions that led them right back to the unfixed switch.
Documents show that on the ignition switch inquiry Barra's lieutenants served as "champions," an industry term to identify a senior executive who marshals internal resources.
Woychowski, a vice president who reported directly to Barra, was the first champion for the probe, run by field engineer Brian Stouffer. Stouffer did not respond to requests for comment.
Federico took over when Woychowski retired in June, 2012. At the time, Federico reported directly to Barra as vehicle line director for small cars.
Stouffer's investigation discovered that the problem was in the ignition switch that could suddenly move into the "accessory" position and disable airbags.
In December 2013, Stouffer's findings were reviewed by two internal committees that decide on product recalls. After two more months of study, the committees approved a recall. GM did not respond to questions seeking the names of committee members.