A number of observers noted smoke coming from the vicinity of No. 3 turret and the aircraft area just forward of it, just before the explosion. Compared with other nations' warships in wartime service, Japanese battleships contained a large amount of flammable materials including wooden decking, furniture, and insulation, as well as cotton and wool bedding. Although she had been modernized in the 1930s, some of the "Mutsu"s original electrical wiring may have remained in use. While fire in the secure magazines was a very remote possibility, a fire in an area adjacent to the No. 3 magazine could have raised the temperature to a level sufficient to ignite the highly sensitive black-powder primers stored in the magazine and thus cause the explosion.