Slowly, the Border Patrol truck cut through the darkness, its headlights illuminating a deserted, sandy road framed by long grass and yellow mesquite trees.

“This area used to be really hot,” said Marlene Castro, a supervisory Border Patrol agent, as she scanned the path ahead for immigrants who had crossed the Rio Grande illegally into the United States. “You couldn’t move. Every time you turned a corner, you’d run into group after group.”

At 10 p.m. on a Friday night, this once-bustling crossing point – a stretch of wild brushland between the Rio Grande and the sprawling Texas border cities of Hidalgo and McAllen– is desolate. The only signs of life, apart from the odd wildcat stalking prey, are Border Patrol agents lurking by the roadside in pickup trucks and SUVs.

Across the Southwest border, the number of immigrants caught crossing illegally into the United States has dropped dramatically. Fewer than 12,200 people were apprehended in March, a 64% decrease from the same time last year, and the lowest monthly number in at least 17 years.