Feb. 24, 2016—Ten automakers investigating the cause behind the exploding airbag inflators made by Takata Corp. have determined it was caused by a mix of three factors, according to a report by Automotive News.

The group, known as the Independent Testing Coalition (ITC), determined that the ammonium nitrate propellant, when exposed to humidity and repeated temperature swings, can cause it to combust violently and rupture the inflator when airbags are deployed, according to the report.

David Kelly, leader of the ITC and former chief of the National Highway Traffic Safety Administration, said that the combination of  the use of ammonium nitrate, the construction of Takata’s inflator assembly and the exposure to heat and humidity are what caused the inflators to rupture.

“You can’t have the energetic disassembly without all three factors,” Kelly told Automotive News. “You have to have all three.”