The recall of potentially defective Takata airbags, one of the largest and most complex auto recalls in the nation's history, may grow even larger.
Officials from the National Highway Traffic Safety Administration said on Thursday that the agency had expanded its investigation beyond the airbags situated in front of the driver and front-seat passenger to include side airbags. It said was also examining all model years, not just older inflaters, for defects that could cause the airbags to rupture violently and spew metal fragments at vehicle occupants. If those airbags are determined to be defective, the regulator said, the already spiraling recall may need to be widened.
More than 19 million cars made by 12 automakers have already been recalled in the United States to fix potentially dangerous airbags. Millions more have been recalled abroad. The ruptures have been linked to eight deaths worldwide and dozens of injuries.
So far, the recalls have been limited to older airbag inflaters — the metal casings that contains the propellant — in front of the driver and passenger. But recently, side airbag inflaters, including those in new cars, have raised concerns.
This summer, regulators asked Volkswagen to provide information about the rupture of a Takata-made side airbag in a Tiguan from the 2015 model year. And last weekend, General Motors recalled almost 400 cars in the United States after Takata informed the automaker that side airbags in those vehicles had failed in tests.
"These have all been brought under the current investigation," Mark R. Rosekind, the administrator at the safety agency, said to reporters on Thursday after a public meeting on the airbag recall.
Gordon Trowbridge, a spokesman for the agency, said that were "within the scope of the investigation — all model years."
It is not known, though, how many vehicles could potentially be affected. Of the 12 automakers with Takata-made airbags, only Chrysler gave an estimate — 25,000 vehicles — but it was limited to those with inflaters similar to the ones in the VW and G.M. cars in question. A spokesman for Chrysler noted that it had not experienced issues with those inflaters. Some of the auto manufacturers said they needed more time to confirm figures. Others declined to comment altogether.
Takata said it was investigating the latest ruptures.
"While we are still investigating the cause of this malfunction, we believe it is unrelated to the previous recalls, which the extensive data suggests were a result of aging and long-term exposure to heat and high humidity," said Jared Levy, a Takata spokesman. "We are cooperating closely with N.H.T.S.A. and the vehicle manufacturers."
Mr. Levy declined to give an estimate of how many cars might be affected if the recall were expanded to include side airbags.
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Also on Thursday, Senators John Thune and Bill Nelson, the top Republican and Democrat on the Commerce, Science and Transportation Committee, sent a letter to Takata demanding documentation about the side airbags.
"Takata has repeatedly emphasized the critical role long-term exposure of vehicles to high heat and humidity may play in ruptures of its ammonium nitrate-based inflaters subject to previous recalls," the senators wrote. "This incident, however, involved a vehicle that was less than one year old."
Automakers are not required by law to install side airbags in vehicles, but during the 1990s many developed and added them in response to rules strengthening side-impact protection.
Officials at the agency said that they still had not determined the root cause of the explosions, but they suggested that the ruptures were most likely related to the ammonium nitrate that Takata used to inflate its airbag.
"We believe that the reason these inflaters are malfunctioning in this way has something to do with the type of propellant Takata is using and how Takata engineers it," said Stephen A. Ridella, director of the safety agency's office for vehicle crashworthiness research, who spoke during the meeting. He said the agency was looking at the way the ammonium nitrate, which is treated with stabilizing chemicals, changes as it ages and burns differently.
Patents filed by Takata show that its engineers have been aware of the potentially dangerous effects of moisture and volatile temperatures on ammonium nitrate for almost two decades and have long struggled to stabilize the compound.
Former Takata engineers have also said that they raised concerns over the use of the compound in the late 1990s but that their concerns went unheeded.
On Wednesday, The New York Times reported that, in 2010, as Takata and Honda assured regulators that the airbag explosions were linked to isolated manufacturing issues, they were also enlisting the help of a top pyrotechnic lab at Pennsylvania State University to determine whether ammonium nitrate might have been at the heart of the problem.
When the study's conclusion in 2012 cast doubt on the use of ammonium nitrate, Takata disputed the methodology, dismissed the conclusion and waited more than two years before sharing the research with regulators.
Frank Borris, director of the agency's Office of Defects Investigation, said that federal investigators' understanding of the nature and scope of the defect had evolved.
"For several years, we believed that manufacturing errors caused the ruptures," he said. "We no longer believe that to be true. Today, the exact cause of the ruptures is unknown, though it seems to be related to environmental conditions that affect inflaters as they age."
Scott Yon, the agency's chief of vehicle integrity, said that the risk of ruptures affected vehicles from five to 10 years old, and possibly older. He said that all seven of the deaths linked to the defect in the United States involved drivers, though it was still unclear why. That could simply be because there is always someone sitting in the driver's seat, or it could be because the airbag inflater is located in the steering wheel, close to the driver, he said. The lone death outside the United States, in Malaysia, also occurred in the driver's seat.