But the mass transfers are raising alarm among immigrant advocates, who were already concerned about the lengthy periods of time migrant children are spending in federal custody.

The roughly 100 shelters that have, until now, been the main location for housing detained migrant children are licensed and monitored by state child welfare authorities, who impose requirements on safety and education as well as staff hiring and training.

The tent city in Tornillo, on the other hand, is unregulated, except for guidelines created by the Department of Health and Human Services. For example, schooling is not required there, as it is in regular migrant children shelters.

Mark Greenberg, who oversaw the care of migrant children under President Barack Obama, helped to craft the emergency shelter guidelines. He said the agency tried “to the greatest extent possible” to ensure that conditions in facilities like the one at Tornillo would mirror those in regular shelters, “but there are some ways in which that’s difficult or impossible to do.”

Several shelter workers, who spoke on condition of anonymity for fear of being fired, described what they said has become standard practice for moving the children: In order to avoid escape attempts, the moves are carried out late at night because children will be less likely to try to run away. For the same reason, children are generally given little advance warning that they will be moved.

At one shelter in the Midwest whose occupants were among those recently transferred to Tornillo, about two dozen children were given just a few hours’ notice last week before they were loaded onto buses — any longer than that, according to one of the shelter workers, and the children may have panicked or tried to flee.

The children wore belts etched in pen with phone numbers for their emergency contacts. One young boy asked the shelter worker if he would be taken care of in Texas. The shelter worker replied that he would, and told him that by moving, he was making space for other children like him who were stuck at the border and needed a place to live.