A sharp dip in violence in Colombia's second-biggest city has attracted a flood of tourists to Medellin's vivid colors, busy cafes and booming nightlife. About 1.4 million people visited Medellin last year, many of them American. Now the surge in tourism has presented officials with a new set of dark challenges, including an uptick in sex trafficking and the killing of tourists and Colombian women after rendezvous on dating apps. The dangers came to a head late last year. Eight American men were killed between November and December, many after meeting local women who are often used as pawns by criminal groups that target foreigners.