The 36-year-old Cuban mechanic's eyes glazed over as he recalled his time at the Siglo XXI holding facility: 50 people sleeping in 3-by-4-yard (-meter) pens, feces overflowing the latrines, food and water always scarce.

Women slept in hallways or in the dining hall among rats, cockroaches and pigeon droppings, as children wailed, mothers reused diapers and guards treated everyone with contempt.

Many migrants who cross into southern Mexico end up in Siglo XXI, Spanish for "21st century," said to be the largest immigration detention center in Latin America. Located in the city of Tapachula, near the border with Guatemala, it's a secretive place off-limits to public scrutiny where cellphones are confiscated and journalists aren't allowed inside.