McALLEN, Texas — Hundreds of young migrants are being kept behind metal wire — the type you’d see on a neighborhood batting cage or a dog kennel — inside the country’s largest immigration processing center.

A Department of Homeland Security official called the facility, known as Ursula, the “epicenter” of the Trump administration’s policy that has separated thousands of children from their parents.

A total of 1,174 children have been taken away from their mothers and fathers in the Border Patrol’s South Texas Rio Grande Valley sector, with many brought to the Central Processing Station in McAllen, Texas, since the policy was announced on May 7, according to Manuel Padilla, the Border Patrol sector chief.

NBC News was part of a group that went behind Ursula's highly secured doors Sunday to see firsthand what migrants go through before separations occur.

The facility has spanned multiple presidential administrations but a new "zero tolerance" policy — which Trump's own top staff and Cabinet officials have explicitly said is causing children to be taken from their parents as part of an effort to deter border crossings — has turned the spotlight on it.

From Ursula, children will be sent to separate facilities run by the Department of Health and Human Services while their parents are sent to a detention center to await prosecution before a federal judge.

U.S. policy dictates that no one of any age should be kept in one of these processing centers longer than 72 hours. But because of the backlog at HHS centers for children, hundreds of minors are forced to stay past that limit.

As of Monday, the HHS had 11,785 minors in its custody, a department official told NBC News.

That number includes "all minors at all shelters and facilities in the unaccompanied alien children program," the official said.

Cameras were not permitted on the Father's Day tour, but the Border Patrol provided handout images of the stark situation: 1,129 migrants were detained in the 77,000-square-foot facility, nearly all of them behind the metal wire.

Mylar blankets, the type marathon runners wrap themselves in after finishing a race, covered the bodies of migrants throughout as they lay on mattresses atop concrete floors.

Migrants rest under mylar blankets at the Border Patrol's Central Processing Station in McAllen, Texas, on Sunday. This photo has been manipulated by authorities to protect identities. U.S. Customs and Border Protection

In the 55,000-square-feet of the facility dedicated to families and unaccompanied minors, detainees were sorted based on age, gender and family status into what the Border Patrol called four pods: one for girls 17 and under, another for boys 17 and under, mothers with children and fathers with children.

Agents are overwhelmed. John Lopez, the deputy patrol agent in charge, told NBC News they lack manpower and the system is strained — and here, they’re only separating less than half of the families so far.

Parents who are separated from their children aren’t taken away until they are brought into processing to leave the facility — only at that moment do they find out if they’ll be prosecuted, instead of taken to an ICE detention center with their children. They receive what is called a “tear sheet” informing them of their fate and how they might find their children again.

For some, agents said, processing comes not with one of the 10 permanent processing agents but with virtual ones — video chats with agents in El Paso, En Centro or Corpus Christi — an effort to provide support for a system they say is understaffed.

Only four social workers were on hand to care for the hundreds of children, a backup system when Border Patrol agents are not prepared or qualified to deal with the challenges that come with caring for a child.

Some children who arrived with parents now find themselves alone in the facility before they are picked up and taken into the custody of the HHS, which cares for unaccompanied migrant children. Others arrived on their own.

Padilla, the Border Patrol chief, said agents in the Rio Grande Valley have allowed families with children under 5 to stay together in most cases.

An advocate who spent several hours in the facility Friday told The Associated Press she was deeply troubled by what she found.

Michelle Brané, director of migrant rights at the Women’s Refugee Commission, met with a 16-year-old girl who had been taking care of a young girl for three days.

“She had to teach other kids in the cell to change her diaper,” the AP quoted Brané as saying. “She was so traumatized that she wasn’t talking. She was just curled up in a little ball."

A total of 1,129 migrants were detained in the 77,000-square-foot facility known as Ursula on Sunday. U.S. Customs and Border Protection

Trump has repeatedly blamed Democrats opposed to his immigration reform proposal, falsely crediting an anti-trafficking law that passed unanimously in 2008 under President George W. Bush for the separations.

The detention of children apart from their parents is a result of the policy mandated by Attorney General Jeff Sessions, and there is no law that requires family separation. As such, congressional action is not necessary to stop it. Sessions has said the intent is to eventually prosecute everyone who crosses the border illegally.

Last week, Sessions gave a full-throated defense of the policy leading to family separations, saying having children does not give migrants immunity from prosecution and citing the Bible as justification.

"Noncitizens who cross our borders unlawfully, between our ports of entry, with children are not an exception," the attorney general said. "They are the ones who broke the law, they are the ones who endangered their own children on their trek."

Jacob Soboroff reported from McAllen, Texas, and Julia Ainsley from Washington.