By Fernanda Santos

New York Times News Service

NOGALES, Ariz. — In a 120,000-square-foot warehouse on the edge of this desert city, Border Patrol agents line up hundreds of children who may have never seen before a doctor for basic vaccinations and other medical care, hand out snacks or join them for a game of basketball under a circuslike tent that doubles as a recreation room.

In a makeshift processing center, the children — all minors caught crossing the border in the Rio Grande Valley of Texas without parents — are housed for as many as three days or more in nine holding pens: boys are separated from girls and older children from younger ones; teenage mothers and their babies stay in a cell of their own.

There is barely room to walk; mattresses line the concrete floor, which also has long bleachers bolted to it. The children are being transferred here from Texas because a similar facility there cannot take any more.

Customs and Border Protection officials said Wednesday that 900 children from Guatemala, El Salvador and Honduras were being held here — the newest arrivals still in the clothes they wore on their trek to the United States, the others clad in white T-shirts and blue shorts, as in a reformatory. On one mattress, a girl barely in her teens wept, her face buried in a soiled stuffed lamb. Nearby, a toddler smiled as she held the hand of a Border Patrol agent taking her for a walk.

As detainees, none of the children are allowed to go outside except to exercise for 45 minutes to an hour a day.

Chief Manuel Padilla, the agent in charge of the Border Patrol’s Tucson Sector, said the agency’s goal was to keep the children “safe,” “healthy,” “nourished” and “clean,” and that a lot had been done “to achieve these priorities,” sometimes in small ways.

When agents noticed that the children were refusing their breakfast burritos, which were made with flour tortillas, the kitchen switched to corn tortillas, like the ones used in Central America.

On Wednesday, the Border Patrol gave reporters a first glimpse of this processing center as well as a similar one in Brownsville, Texas, both focal points in the national debate over the sudden stream of unaccompanied minors crossing illegally into the United States. From here, the children will be sent to juvenile detention facilities around the country, where efforts will be made to release them to relatives in the United States on the condition that they cooperate with deportation proceedings.

But the swelling number of arriving youths — many of them making perilous journeys to flee gang violence in their native countries — has presented the Obama administration with political and humanitarian predicaments, and started to dominate the nation’s conversation over immigration reform.

Rep. Candice Miller, R-Mich., the vice chairwoman of the House Homeland Security Committee, called on the administration Wednesday to deploy the National Guard along the Texas-Mexico border.

Also on Wednesday, two Texas legislators — Sen. John Cornyn, a Republican, and Rep. Henry Cuellar, a Democrat — sent a letter to Homeland Security Secretary Jeh Johnson demanding answers to a series of questions about how the children were being handled: Are they tracked upon their release from custody? Does anyone check to see if they have criminal records or gang affiliations? What measures does Homeland Security take to “help ensure that they do not end up in the hands of predators or sex offenders?”

Acknowledging the dangers of the illegal border crossing, the letter also asked, “What percentage of unaccompanied alien children apprehended at the southern border this fiscal year were victims of crime or exploitation while traveling through Mexico?”

In Nogales, the logistical challenges of caring for the children are clear. There are three portable restrooms in each pen, and 60 showers in five trailers like the ones used by the Federal Emergency Management Agency in disaster areas. In the holding pens, there is no source of entertainment other than televisions with seemingly no sound or improvised games of soccer played in cramped corners.

At the processing center here, Art Del Cueto, president of the Tucson chapter of the union for Border Patrol agents, said the agents were under pressure and felt overwhelmed by the unexpected demands they were facing.

“Catching illegal aliens is part of the job,” he said in an interview. “Processing is part of the job. But baby-sitting is not part of the job, and that’s what a lot of the agents have been doing.”

The agents are the children’s most frequent escorts and often the ones to explain to them what is happening and what lies ahead. Many of the agents are bilingual, and Del Cueto said it was fair to assume that many of the children do not comprehend the gravity of their situation.

Speaking to reporters at the start of the tour of the facility here, Padilla said that his agents were working to fulfill a “humanitarian mission” while carrying out their task of securing the border.