TAPACHULA, Mexico - Secretary of Homeland Security Jeh Johnson toured a facility Wednesday in Nogales, Arizona, that is holding 900 children caught entering the United States illegally.
Most immigrants begin their perilous journey in Central America, crossing through southern Mexico.
People desperate to reach the United States cling to this freight train - a dangerous 1,200-mile trip north through Mexico.
They call the train "La Bestia" - The Beast.
Byron Solares, of Guatemala, fell off The Beast in 2009 and lost a leg.
"I was running away from robbers," Solares said, speaking in Spanish.
Gangs are on the train too, and kidnap thousands of riders each year, according to Mexico's National Human Rights Commission.
Solares recovered at a shelter for injured migrants in Tapachula. He decided to stay and help others.
"I warn people to think hard about leaving their countries," he said.
But a worker told us this year, the shelter has housed more than 100 people a month - twice as many as last year - with few additional resources.
Across town, a children's shelter is full. It's run by Ramon Verdugo.
"They come here hoping for safe place," he said.
Most are from Honduras, which has the highest murder rate in the world. A U.N. survey of Central American minors who entered the U.S. illegally this year found 58 percent pointed to violence as the reason for leaving.
Many cross the porous Mexican-Guatemalan border on rafts.
"Immigrant children are fleeing exploitation and misery," Verdugo said. "That's exactly what they find on this side, too."
He says minors who stop here to make money for the rest of the trip often become victims of child labor or sexual abuse.
For young and old, the risks of the journey are daunting, but the draw north is much stronger.