How fentanyl changed the game for Mexico's drug cartels

One Sunday morning in May, after leaving a restaurant in western Mexico, Sergio Emmanuel Martà­nez, a new customs director at the country's largest port, was kidnapped. The next day, he was found dead beside a motorway, making him the fourth customs official at the port of Manzanillo to be murdered in less than two years. Manzanillo is a bustling centre of global commerce but it is also an entry point for chemicals from China that are used to make the synthetic opioid fentanyl. Like other Pacific coast hubs, its importance to the drugs business has risen sharply with the fentanyl…