The mountainous region is crisscrossed by back roads and footpaths. And residents and others speculated that entrenched corruption among government officials who have abetted smugglers and migrants would not be eliminated anytime soon.

But the number of government roadblocks along some main highways in southern Mexico has increased, local residents say, and the migration authorities and security forces working with them seem to be more thorough in checking passing vehicles for undocumented migrants.

The authorities have also started the more ambitious work of combating the smuggling operations that are responsible for escorting many, if not most, migrants north.

Migrants’ advocates have warned that the crackdown could invite human rights violations, concerns that were underscored by the recent shooting death of a 19-year-old Salvadoran migrant who was riding in a truck bound for the United States border. Witnesses told investigators that men dressed in police uniforms and driving a police vehicle opened fire on the truck after it passed through a migration checkpoint in the state of Veracruz and sped away.

While National Guard members are still scarce in the southern border region, military forces, some of them outfitted with National Guard arm bands, have been newly mandated to conduct night patrols, and to question the occupants of passing vehicles and inspect their cargo.

Immigrants’ advocates anticipated that the increasing presence of security forces would continue to drive down the number of people trying to migrate north, but they expected that in time, the flows would rebound, perhaps through more remote and dangerous migratory pathways.

“It’s going to be like when Trump became president, and the rate of migration went down for several months, but then went back up again,” said David Tobasura, a Chiapas-based immigration consultant for the American Friends Service Committee. “This is not going to stop migration. Surely in a few weeks or months, it will go back up. It’ll be the same as it was before.”