An ex-cop turns to gun trafficking, spreading his deadly wares to criminals and terrorism suspects nationwide, including Minnesota. Hundreds of his weapons remain in unknown hands. There were 501 guns in all, the government says — revolvers and pistols, Glocks and Rugers, a few rifles, too — a giant cache of firearms suitable for sport or self-protection. Or felonies. Or terrorism. Five hundred and one guns, tied to one man — 35-year-old Mark Nelson, a former Columbus cop who masterminded a conspiracy that flooded the streets with weapons. Nelson insists he didn't knowingly do anything wrong. But prosecutors say he...