His language allowed for the possibility that the assailants were not legitimate police officers. In Mexico, criminals have been known to disguise themselves in the uniforms of the nation’s security forces to conduct crimes, though corruption also permeates Mexico’s police forces, and some police personnel have developed close ties with organized crime.

As the three men beat up the driver of the truck, the other migrants ran away, Mr. Winckler said. The men in police uniforms hauled the driver away, leaving behind the dead migrant and her two wounded companions, one of whom had been shot in a hand and the other in his leg, Mr. Winckler said, citing witness testimony.

One of the wounded migrants told The Washington Post that the occupants of the patrol car had first shot at the migrants’ vehicle from behind, before pulling ahead and shooting through the windshield. The wounded migrant, who was recovering in a hospital in Veracruz, said he saw all three men wielding weapons.

The deployment of security forces by the Mexican government, after the deal announced on June 7 with the Trump administration, was supposed to be concentrated in Mexico’s southern states. Officials initially promised to send 6,000 members of the newly formed National Guard to the southern border region to help migration officials intercept undocumented migrants entering from Guatemala.

But the mobilization has been halting at best. In recent days, officials said that only a portion of those 6,000 would be members of the National Guard, with the rest drawn from other security forces, and that only about 40 percent of them would be sent to the southern region.

Still, the deal has already resulted in more government checkpoints along major roadways leading from the border with Guatemala into the interior of the country, and more aggressive patrols.

Human rights experts, migrants’ advocates and security analysts have warned about possible hazards, such as human rights violations, that could be caused by the quick deployment of the security forces, which may not have received proper training in migration enforcement.