Chinese investigators have ruled that the deadly warehouse blasts that killed 165 people in the port city of Tianjin in August were triggered when the stored chemical nitrocellulose became too dry and caught fire.

The investigation team said Friday that flames spread among stocks of nitrocellulose, a flammable compound used as a binding agent with medical applications and as an ingredient in lacquer. The flames then spread to stores of highly combustible ammonium nitrate, touching off massive explosions.

The 165 people killed in the blasts included 99 firefighters responding to fight the fire and 11 police officers.

Investigators previously have ruled that regulators failed to enforce safety rules, including keeping hazardous materials a proper distance away from residences. Several dozen people have been arrested.