Most of the 149 migrants being taken across the bridge on Thursday had crossed into the United States from Reynosa, a border city in northern Mexico, where they had been detained by U.S. Border Patrol officers. They were then flown 600 miles to El Paso, Texas, where they were put on buses, driven to the border and walked to the bridge.

None were informed they were being sent back into Mexico.

As they walked across the bridge connecting El Paso to Ciudad Juárez, it dawned on them that everything they had risked on their journey — their lives, the well-being of their children, the loans they had bankrupted themselves to take out to be smuggled into the United States — was falling apart…

“They deceived us because in the United States they never told us that they were going to deport us,” Mr. Bautista said.