The attack shattered the sense of idyll in this remote community of American-style ranch houses, where for generations life carried on largely unchanged, even in the past decade as a rising tide of drug-related violence in Mexico claimed about 200,000 lives, according to government figures.

It also pierced the illusion that the families’ American citizenship protected them from the violence. They believe they were targeted by gunmen, and doubt the government’s version of events that the assailants mistook their SUVs for those of a rival gang. Even if they aren’t sure, staying means risking more lives.

Most of the people who left the community this weekend aren’t returning, said Mr. Langford. “One after another, they decided to leave,” he said in a telephone call from Arizona. “The safety of the children and family are No. 1.”

Not everyone can leave. Adam Langford, who has twice been mayor of Bavispe, the municipality where La Mora is located, is married to a Mexican national. But he estimates that at least half the residents will depart—he hopes temporarily, if security can somehow be restored. “We are going to be by ourselves.…It’s going to be pretty sad.”