We had just walked into the fentanyl lab when the cook poured a white powder into a stockpot full of liquid. He began mixing it with an immersion blender and fumes rose from the pot, filling the small kitchen.

We wore gas masks and hazmat suits, but the cook had on only a surgical mask. He and his partner had rushed here to fulfill an order for 10 kilograms of fentanyl. While one sniff of the toxic chemicals could kill us, they explained, they had built up a tolerance to the lethal drug.

But then, the cook jerked back.

“It really hit me,” he said, looking dazed. “I need to take a breather.”

The young man rushed out of the room.

In September, a war broke out within Mexico’s Sinaloa cartel. Fighting between the rival factions has terrorized the northwestern state of Sinaloa in the months since, leaving hundreds dead and causing a billion dollars in economic damage, business leaders say. The Mexican government responded by sending in a swarm of soldiers and making a slew of arrests.