This week, seven federal officers died in a gun battle with cartel henchmen when they tried to enter a house in Culiacán, Sinaloa, a city notorious for its traffickers. The officers had been sent to the city, along with 2,700 other soldiers and agents, to track down a reputed drug kingpin believed to have ordered the assassination of the acting federal chief of police, who was killed in Mexico City on May 8.

Image In Villa Ahumada, Mexico, on May 18. The night before, dozens of gunmen killed six people in the town, including two civilians who were together in a pickup truck, and abducted others. Credit... El Heraldo de Chihuahua

When the police arrived, banners were hung in the city taunting the officers and saying the reputed kingpin, Arturo Beltrán Leyva, reigned supreme in Culiacán.

In Villa Ahumada less than two weeks after the massacre, people remained so cowed that even the mayor and his police commissioner declined requests to be interviewed. When asked who the gunmen were and why they had come, most of the residents who were interviewed shook their heads and whispered that spies were everywhere. In private, however, some acknowledged that the town had long been home to narcotics traffickers in league with a reputed drug dealer, Pedro Sánchez Arras.

Frightened residents, who did not want to be identified, said Mr. Sánchez’s agent in the town was Gerardo Gallegos Rodelo, a 19-year-old tough guy who went around with an armed posse. It was rumored that he and Mr. Sánchez had links to a drug cartel in Ciudad Juárez that is controlled by the Carrillo Fuentes family. Law enforcement officials did not confirm the claim.

Several residents said Mr. Gallegos and Mr. Sánchez had also seemed to enjoy good relations with the local police. People shrugged and tolerated the arrangement. The town was peaceful, after all, some said. It seemed best to leave well enough alone.

“Wherever you are in Mexico these days there are drug dealers, not just here,” explained Raúl Moreno, 64, a day laborer. “They didn’t bother anyone. No one bothered them.”

The trouble started, people here say, when Mr. Gallegos was killed in a shootout with a group of reputed gangsters in Hidalgo del Parral, in the southern part of Chihuahua State, on April 6.