TAPACHULA, Mexico — 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 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.

"They threw us in there like little animals," a Honduran woman said.

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.

The Associated Press was denied access, and the National Immigration Institute, which oversees the facility, did not respond to requests for comment. But about 20 people interviewed by AP including migrants, officials and rights workers who've been inside, described it as sorely overcrowded and filthy, and alleged repeated abusive treatment by agents tasked with running it.

Washington has demanded Mexico slow a migratory surge of mostly Central Americans fleeing poverty and violence but also Cubans, Haitians and Africans , and President Donald Trump has said threatened tariffs on Mexican imports may be revived if it doesn't get results. As President Andrés Manuel López Obrador puts in motion a still-vague plan to crack down in response, observers fear an already overtaxed Mexico is woefully unprepared for the prospect of more arrests.

"If more people are detained, there is not the corresponding infrastructure to handle it," Edgar Corzo of the governmental National Human Rights Commission said Thursday during a tour of southern Mexico ahead of an anticipated, 6,000-strong National Guard deployment to help police immigration.

As of late April there were more than 2,000 migrants in Siglo XXI, according to the commission, over double its 960 capacity. Hundreds were moved to an improvised camp and that was down to 1,230 as of last week, Corzo said. Another facility in Tuxtla Gutierrez with a capacity of 80 was holding 400.

"I cannot imagine putting 100 more or hundreds more at the Siglo XXI. ... The migratory stations are not prepared to respond to greater capacity because they have been overwhelmed," Corzo said.

Most of those interviewed for this story spoke on condition of anonymity for fear of reprisals. But from them a picture of the hermetic Siglo XXI emerges.

It's a prisonlike compound with 16- to 30-foot walls, control towers, security cameras and high-ceiling caged areas where guards patrol above the migrants. There is a punishment cell called "the well" which the government has promised not to use, though the Fray Matias de Cordova Human Rights Center, one of the few NGOs allowed inside, has not been able to verify that.

Cuban migrants walk on the streets of Tapachula, Chiapas state, Mexico on May 28, 2019. The group says they were singled out by authorities as the ringleaders of the mass break out that happened in April, when more than 600 migrants escaped from the Siglo XXI detention center. Marco Ugarte / AP file

Crossing through the barred entryway, one emerges onto a patio and a sort of loading dock where people come and go on buses. Migrants arrive after being swept up in raids or tricked into thinking they're just going to have their documents checked, several ex-detainees said. Shoelaces, belts and phones are taken away, though extra food, a cigarette or a phone call can be obtained — if you can pay for it.

The U.N. refugee agency and other organizations criticized detention centers in Mexico even before the current crisis, saying migrants are held in substandard conditions and regularly extorted. They have called for detentions to be the exception, and to be completely eliminated for minors. Yet children continue to fill up the centers, even after a girl died in Mexico City under unclear circumstances.

A 29-year-old Honduran woman named Graciela said she didn't sleep at Siglo XXI for fear of rumors that her 7- and 9-year-olds could be taken from her. Agents pressured her to accept deportation, but she refused. The kids were scared.

"They said to me, 'Let's go!' 'Why are we here?'" said Graciela, who was now free and waiting for her refugee claim to be processed. "Sometimes we all cried."

Julio, a 15-year-old Cuban, was separated from his parents though they were all within Siglo XXI.

"I panicked," his mother said of her fear during a detainee riot. "I cried, I begged them to tell me if he was OK, but nothing. I went five days without seeing him."

Foreign Relations Secretary Marcelo Ebrard acknowledged recently that migration facilities in the south have long been neglected and are "well below standards." Then-immigration commissioner Tonatiuh Guillén, who resigned Friday at López Obrador's request, allowed in a recent interview that "they have a very severe model of control."

López Obrador says human rights continue to be the cornerstone of his immigration policy, and officials are studying options for building new and more open facilities.

Migrants at Siglo XXI quickly learn there are two ways out: deported by bus, or released with papers documenting a refuge request. But even that's no guarantee. The mechanic said agents tore up his asylum document in front of him for no apparent reason.

The federal government has also acknowledged that the National Migration Institute is one of Mexico's most corrupt institutions. More than 600 employees have been purged and new agents have sent to Siglo XXI, though details are unknown.

Salva Lacruz of Fray Matias said Siglo XXI remains in "negligent, irresponsible and racist" hands, and in disregard of directives from Mexico City.

In their desperation some detainees recently found a third way out: rioting and escaping.