The F.B.I. director, James B. Comey, on Monday said that in the past week law enforcement authorities had arrested 281 pimps and recovered 168 child prostitutes in a nationwide sex trafficking sting. The arrests and recoveries were made in 106 cities, and roughly 10 percent of the prostitutes had been missing from child protective services, the authorities said. In Phoenix, 21 pimps were apprehended, and in Denver 18 prostitutes were recovered, the highest numbers in the operation. More than a dozen children were recovered in other cities, including Chicago and Cleveland.