"[I]t’s best for people to stay back, 20 or 25 feet"

The agency’s explanation of a Category 3 source that hasn’t been dispersed by fire or explosion reads: “This source, if not safely managed or securely protected, could cause permanent injury to a person who handled it or who was otherwise in contact with it for some hours. It could possibly — although it would be unlikely — be fatal to be close to this amount of unshielded radioactive material for a period of days to weeks.”

The Texas health department says the cylinder is stamped with the words “danger radioactive” and “do not handle” along with a radiation warning symbol. Anyone who sees it is advised by the department to keep away and notify local law enforcement.