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The surge in Central Americans seeking asylum in the United States is straining facilities across the Southwest border, with Border Patrol processing facilities this week handling thousands of people in excess of the system’s capacity. President Trump threatened to close the border next week if Mexico did not halt “all illegal immigration” into the United States.

El Paso, the Texas border city where the president held a rally in February calling for his wall with Mexico, is emerging as a flash point. Border Patrol agents in the city have begun holding migrant families in an area under a bridge, surrounded by fencing and razor wire.

Photos of the conditions drew attention this week on social media and in news reports. Here’s a look at where the migrants came from, and how they came to be detained under the bridge.