General Motors Chief Executive Mary Barra on Tuesday vowed to find out whether GM engineering employees executed a coverup or were merely incompetent in failing to recall a defective part linked to 13 deaths.

Testifying before a House Energy and Commerce Committee panel, Barra said an independent investigation she ordered should provide that answer and explain why the company waited years to fix a faulty ignition switch in multiple models.

GM only recently recalled 2.6 million vehicles to fix the switch even though the automaker learned of the problem more than a decade ago.

The fix took "way too long," she told lawmakers. "We will make changes and we will hold people accountable."

But Barra — a lifelong GM employee named to head the automaker in January — repeatedly deflected questions about a coverup and provided no specifics about who nixed the recalls and why. She said the automaker has hired former U.S. attorney Anton "Tony" Valukas to lead an inquiry to find those answers.

Previously Valukas investigated the collapse of investment bank Lehman Bros. in 2008.

David J. Friedman, the acting administrator of the National Highway Traffic Safety Administration, followed Barra at the hearing and was immediately challenged by panel members asking why the agency didn't force a recall years ago.

Friedman said his agency conducted a proper review of the available data it had about crashes involving the vehicles. He said GM withheld key information that might have led NHTSA to force earlier recalls.

"Our ability to find defects also requires automakers to act in good faith and to provide information on time," Friedman said.

He noted that GM now has provided the agency with new information "definitively linking" the faulty ignition switch with the failure of air bags to deploy in accidents and other data. The agency is investigating whether GM withheld information it was required by law to provide to regulators, he said.

The recalled models included 2003 to 2007 Saturn Ions, 2007 to 2010 Saturn Skys, 2006 to 2011 Chevrolet HHRs, 2006 to 2010 Pontiac Solstices, and 2005 to 2010 Chevrolet Cobalt and Pontiac G5 models.

Barra conceded many of the lawmakers' criticisms and apologized to crash victims and their families. She called documents that showed the automaker resisted recalling the cars because of cost issues "disturbing."

"That is not acceptable," Barra said. "Today ... if we know that there is a safety defect on our vehicles, we don't look at the cost but at the speed at which we can fix the problem."

The ignition switch in this case can unintentionally turn off the vehicle and disable key functions, including the air bags that protect occupants in crashes.

Rep. Tim Murphy (R-Penn.) asked whether GM purposely withheld information about problems with the ignition switch and other safety issues when it was negotiating the terms of its bankruptcy and federal bailout in 2009.

Barra said she was not aware of any effort to hide information about potential liabilities, "but I can't speak to every single person."

Rep. Joe Barton (R-Texas), an industrial engineer, asked Barra why the company used an ignition switch that did not meet company specifications.

"Why in the world would a company with the stellar reputation of General Motors purchase a part that did not meet its own specifications?" he said.

Barra responded: "I want to know that as much as you do."

When pressed on whether GM now would purchase parts that don't meet specifications, Barra said in some cases it would if the part meets safety, functionality and durability standards among others.