Last year, a 20-year-old named Christian Michael Mackey arrived at the Phillips 66 gas station in Grand Prairie, Texas, hoping to sell his AM-15 rifle to make some quick cash. He’d said he wanted to buy a more powerful gun, something that could stop what he called a “hoard of you know what.” Mackey told an online group chat he’d started looking at Nazi websites at around 15-years-old, when he began spending hours on white nationalist message boards and talking to other extremists on Instagram and encrypted messaging apps like Telegram. Five years later, he was active in a network...