MEXICO CITY—As many as 50 people are missing after setting out on three-hour car trips this year between Mexico’s industrial hub of Monterrey and the border city of Nuevo Laredo on a well-traveled stretch of road local media have dubbed “the highway of death.” Relatives say family members simply vanished. The disappearances, and last week’s shooting of 15 apparently innocent bystanders in Reynosa, suggest Mexico is returning to the dark days of the 2006-2012 drug war when cartel gunmen often targeted the general public as well as one another. “It’s no longer between the cartels; they are attacking the public,” said activist Angelica Orozco. As many as half a dozen of those who disappeared on the highway are believed to be U.S. citizens or residents, though the U.S. Embassy could not confirm their status. One, José de Jesús Gómez from Irving, Texas, reportedly disappeared on the highway on June 3. …