NEW BRAUNFELS — Off a dirt road connected to ever-flowing Interstate 35, a little metal sign on a wooden fence is the only indication of what lies ahead. Nearby, Buckley Powder, a mining and construction supply company, stores large quantities of ammonium nitrate, the source of the explosion at a fertilizer depot that killed at least 14 people and injured hundreds more last month in West.

In 2012, according to state records, Buckley Powder had as much as 90,000 pounds of ammonium nitrate in bins at this Central Texas plant — stored, according to Howard Wichter, Buckley’s chief financial officer, under conditions in which “nothing can happen to it.”