The bomb would be made with a new variant of ammonium nitrate fertilizer that had been touted as non-detonable. For Pentagon scientists, such as Best, the search for a fertilizer that doesn’t explode has been a Holy Grail-like quest. Ammonium nitrate, which packs a fearsome punch, is used in more than 60 percent of the Taliban’s bombs. It’s also essential to farming; without it, thousands of Afghans and Pakistanis would starve.

This spring, an engineer at Sandia National Laboratories announced that he had found a special additive that blunted the fertilizer’s blast without damaging crop yields. Sandia trumpeted the breakthrough in a news release. “Fertilizer that fizzles homemade bomb could save lives around the world,” the federally funded research laboratory promised.

But Sandia can’t test bombs, because it is a tightly regulated activity. Best would give the formula its first official try. He whipped up a batch of the Sandia fertilizer and took it to his test facility, one of the more secretly guarded places in Washington. The range is at the end of a narrow, two-lane road and is surrounded by woods, family farms and a church advertising a “weekend Shad bake.” There’s no sign at the base’s entrance, just a big metal fence, lots of barbed wire and several video cameras.