Megan Greenwell’s grandfather served in the US military after becoming a naturalized citizen in 1943. Were his personnel records among the nearly 18 million files destroyed by fire in 1973 at a branch of the National Personnel Records Center? In this superlative story for Wired, Greenwell attempts to find out.

So she and her colleagues climbed one more flight of stairs, to a door that opened into the sixth and top floor. She remembered that this was where the older military records were kept, the ones from World War I, World War II, and Korea, but she hadn’t been up here since orientation. Now, as she pulled open the door, she saw the cardboard boxes neatly stacked on metal shelves as far as the eye could see.

They were on fire.

Had the group gone up a staircase on the periphery of the building and not the central one, Trieschmann likely would have seen only a thick cloud of smoke. Instead, she witnessed the earliest stage of a blaze that would occupy hundreds of firefighters for days.