In South Texas over 4,000 migrants are crammed into one facility, four times its capacity before the global health crisis.
Most are unaccompanied minors.
The Biden administration allowed reporters to tour this processing facility in Donna on Tuesday.
Acting executive officer for U.S border patrol in the region, Oscar Escamilla, said many of the children have gone through long and harrowing journeys on their own.
"These kids cross by themselves. Obviously, the parent pays a fee to the smuggler, the cartel member or the smuggler at this point will bring the kid over... will place them in a raft and say, 'OK, go.When you get to the other side, they'll explain to them there's going to be an officer, they'll explain what we wear and tell them, turn yourselves into them."
A top U.S. official said on Tuesday he expects more than a million migrants to arrive at the U.S.-Mexico border this year, a growing humanitarian challenge for President Joe Biden, as more people are driven to the border by poor economic conditions, natural disasters, and hope to stay in the U.S. under his administration.
The Donna facility footage shared with Reuters showed young children sitting inside a playpen, some as young as 3 years old.
More than 2,000 unaccompanied children have been held here for longer than a legal limit of 72 hours.
Escamilla said these kids slip through the cracks of the immigration system.
"There's a little girl that I talked to a little while ago. She said that she had lost her mom and that she doesn't have a father... I asked her, 'What state are you going to? What's your final destination?' She said, 'I don't know. All I know is it snows there. It's all I know.'"
The U.S. Department of Homeland Security said in a statement they were "working around the clock" to quickly move children out of the packed border facilities.
The government Health and Human Services refugee office did not immediately respond to a request for comment.