There are two main possibilities. First, the cylinder might well have been properly cleaned and tested, but subsequently became contaminated in a way that was not detected before the fire occurred. Sometimes other elements of the oxygen filling or containment system become contaminated, and they, in turn, contaminate the cylinder; sometimes a valve is improperly installed into the cylinder or an improper lubricant is used; sometimes contamination occurs during use—the possibilities are numerous. Or, second, the cylinder was not cleaned and tested properly in the first place, and a contaminant was still present to serve as fuel for the fire. One thing is certain: An oxygen-fed fire cannot start without oxygen, an ignition source and a contaminant—so contamination must have been present.