HMC Honda Motor Co Ltd

A group of 10 auto makers probing the cause of rupturing Takata Corp. air bags pointed to a volatile chemical and other factors as the main culprits, according to findings released Tuesday.

The coalition of auto makers found ammonium nitrate lacking a desiccant that absorbs moisture is among the root causes of Takata air bags rupturing, a safety problem linked to 10 deaths and dozens of injuries globally. Takata has used the chemical as a propellant in air-bag inflaters.

The group also blamed two other factors for explosions: prolonged exposure to high temperatures and moisture, and manufacturing problems that failed to prevent moisture from seeping into air-bag inflaters under those conditions. Moisture can cause ammonium nitrate to destabilize, leading to explosions during crashes.

Takata, auto makers and government officials for years have been unable to pinpoint the root cause for the ruptures, which can spray shrapnel in vehicle cabins. Still, the auto makers' findings are broadly consistent with contributing factors that Takata and government officials have already highlighted.

A Takata spokesman said the findings were consistent with the company's own testing and work done on the company's behalf by a German research firm showing that older inflaters with prolonged exposure to heat and humidity are among the significant factors leading to ruptures. Takata is cooperating with the auto makers' investigation, the spokesman said.

A spokesman for the U.S. National Highway Traffic Safety Administration said the agency received and was reviewing information about the auto makers' findings. Under the terms of a November settlement, all of Takata's inflaters using ammonium nitrate must eventually be recalled unless the Japanese supplier can demonstrate their long-term safety, the NHTSA spokesman said.

The auto makers, in the midst of unprecedented recalls of millions of vehicles featuring the problematic air bags, said the findings were the first in a multi-step investigation that would also examine air bags being used as replacement parts and how they are expected to perform.

The initial findings cover 23 million of the roughly 28 million air-bag inflaters currently being recalled in the U.S. by a dozen auto makers, the car companies' group said. That excludes an additional five million inflaters recalled earlier this year.

Overall, auto makers are currently recalling more than 24 million vehicles in the U.S. on account of Takata air bags.

"Identifying this multivariate root cause is an important first step," David Kelly, a former top U.S. car-safety regulator coordinating the investigation for the auto makers' group, said in a statement. He said the group would continue work toward a comprehensive understanding of the air-bag ruptures.

The coalition includes BMW AG, Fiat Chrysler Automobiles NV, Honda Motor Co, Ford Motor Co., General Motors Co., Mitsubishi Motors Corp., Mazda Motor Corp., Nissan Motor Co., Fuji Heavy Industries Ltd.'s Subaru and Toyota Motor Corp. The group tapped aerospace and defense firm Orbital ATK Inc. to conduct 20,000 hours of testing and analysis by engineers, scientists and technicians, the car makers' coalition said.

The testing was complicated by varying inflater designs in different vehicles but still pointed to the three-factor root cause for air-bag ruptures, said Bob Wardle, senior director of technology programs for Orbital ATK's propulsion systems division.

Write to Mike Spector at mike.spector@wsj.com

(END) Dow Jones Newswires

February 23, 2016 15:45 ET (20:45 GMT)

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