In another development on Tuesday, a group of 10 automakers said the cause of Takata (TKTDY) airbag ruptures linked to 10 deaths and more than 100 injuries is a combination of exposure to humidity, design and manufacturing issues, and use of the volatile chemical ammonium nitrate.

Reuters reported on Monday that the National Highway Traffic Safety Administration (NHTSA) continues to investigate whether the recalls of nearly 29 million defective Takata inflators in the United States should be expanded to include another 70 million to 90 million inflators with ammonium nitrate.

A Senate Commerce, Science and Transportation committee report cited a series of internal Takata documents generated over the past 12 years that showed company officials argued data on inflator quality and testing was manipulated to disguise problems. A redacted 2013 document released by the committee indicates an unidentified Takata manager told the company’s senior vice president of quality assurance that proposed limitations to the scope of a 2013 airbag recall might be “a violation of our moral obligation to protect the public.”

U.S. Senator Bill Nelson cited the Monday Reuters report on the Senate floor, and said it was “puzzling” that NHTSA has allowed Takata to continue production of ammonium nitrate-based inflators indefinitely.

“Why aren’t they taking a more aggressive approach? And what’s going on after all of these inflators, based on what we see with ammonium nitrate, have been exploding?” asked the Florida Democrat, reiterating calls by senators for action on the issue.

The current recall may have to be redone, Nelson said, “because auto manufacturers are installing new live grenades into people’s cars as replacements for the old live grenades.”

Takata spokesoman Jared Levy said issues about testing and data handling raised by Nelson and a Democratic staff report “are entirely inexcusable and will not be tolerated or repeated.”

“Issues with validation testing of the original phase stabilized ammonium nitrate inflators are not the root cause of the field ruptures…but these issues are totally incompatible with Takata’s engineering standards and protocols,” Levy said.

NHTSA in a statement on Tuesday cited a consent order issued last November under which “all Takata ammonium nitrate-based inflators must eventually be recalled unless Takata can establish their long-term safety.”

The agency has repeatedly said that some replacement inflators “may be effective for several years, but not for the full life of your vehicle, and therefore may also have to be replaced.” NHTSA has noted that all deaths and injuries reported took place in inflators at least 7 years old.

Former managers interviewed by Reuters described “chronic” quality failures at Takata’s North American inflator plants, an assessment reflected in dozens of company emails and documents dating back to 2001. Those problems, the former managers said, make it difficult for the company and regulators to pinpoint which inflators, among tens of millions, pose a danger.

When exposed to moisture, ammonium nitrate, which is used to inflate the air bag, can cause the inflator to rupture with deadly force, spraying shrapnel into vehicle occupants.

The review by the automaker-backed Independent Testing Coalition and a Utah-based team from Orbital ATK found the ruptures were caused by a combination of three factors: ammonium nitrate propellant without moisture-absorbing desiccant, long-term exposure to repeated high-temperature cycling in the presence of moisture and “an inflator assembly that does not adequately prevent the intrusion of moisture.”

In a statement on Tuesday, Takata said those findings were “consistent” with its own and those of Germany’s Fraunhofer Group, which was hired earlier by Takata to conduct additional testing.

David Kelly, a former NHTSA acting administrator who is coordinating the coalition’s activities, briefed NHTSA and congressional staff members on Tuesday on the findings. A NHTSA spokesman said the agency was reviewing the findings.

Kelly said the group planned to investigate newer inflators with drying agents, along with replacement airbags.

The study only covered inflators that do not have a drying agent, or desiccant. Most of the inflators with ammonium nitrate that have not been recalled have the drying agent.

The consortium, which includes Honda Motor (HMC), Toyota Motor (TM), Ford Motor (F), General Motors (GM) and Fiat Chrysler Automobiles (FCAU) will next investigate the performance of all inflators being used as replacement parts for current recalls.