The tank container was manufactured on 13 December 1973 by a workshop from Bilbao, and at that time it did not meet the requirements for carrying flammable liquids, since it lacked emergency pressure release valves. Therefore, the tank had been used to carry other substances, some of which were highly corrosive. Tests on the remnants of the steel tank revealed microscopic stress cracks consistent with corrosion caused by previous loads of improperly overpressurized anhydrous ammonia. Combined with whether the tanker suffered an impact that caused additional structural damage, these factors likely led to the almost instantaneous rupture of the tank when the flames flashed back into the tanker. Even without safety valves, a structurally sound and properly filled tanker should have been able to maintain structural integrity in a fire long enough to at least allow nearby people to escape.