An unprecedented number of unaccompanied migrant children are at risk of spending the rest of their childhoods in federal custody, CBS News learned in an exclusive interview with the head of the Office of Refugee Resettlement (ORR), the agency that cares for these children.

The federal government is required to pursue "prompt and continuous efforts toward family reunification" of unaccompanied migrant children, according to a landmark court settlement, but for thousands of kids in ORR care, that reunion may never happen.

"Unfortunately, I have well over 4,000 of those children in my care at this time at the Office of Refugee Resettlement," the director, Jonathan Hayes, told CBS News in June. "So conceivably someone could come into our care at 15 years old and not have an identifiable sponsor in the United States and remain with us for a few years."

On their 18th birthdays, many of the children will be taken from ORR's youth holding facilities, referred to as shelters, to adult detention centers operated by Immigration and Customs Enforcement (ICE).

The number of children in this group has risen sharply in recent years, an "alarming" and "deeply concerning" trend, according to three former agency officials who spoke with CBS News.

Children in ORR custody are labeled internally as belonging to one of four groups: Category One children have an identified parent or legal guardian — referred to as a sponsor — in the U.S.; Category Two kids may end up with a relative; Category Three children have potential sponsors who identify as distant family or close family friends.

The children who may be stuck in federal custody — Category Four — have no identifiable sponsor, according to the government.

As of June, Category Four children represented roughly one-third of all kids in ORR care, a far greater portion than in past years, according to former ORR Director Bob Carey, who served during the last two years of the Obama administration.

"It's deeply concerning. It's a significant increase from what we saw during the Obama administration. I think the numbers were really small, I would think under 10 percent," said Carey, who is now a policy adviser at the nonprofit Exodus Institute.

A February 2016 report by the Government Accountability Office called the use of Category Four designations "rare."

Another former ORR official, who asked not to be named, called the sudden change troubling. "Having a third of kids be CAT 4, there's something that's strange about that," the former ORR official said. "That's alarming to me, particularly because the system was never designed for long-term care."

A landmark 1997 court settlement known as the Flores Agreement requires the federal government to try to unite unaccompanied migrant children with relatives as quickly as possible. However, those efforts may be harder to fulfill than in previous years, according to Mark Greenberg, former Acting Assistant Secretary of the Administration for Children and Families, which oversees ORR. Greenberg said potential sponsors may be too afraid to come forward, a view shared by Carey and the other former ORR official.

All three said they've long feared that the Trump administration's immigration policy changes would have a "chilling effect" on potential sponsors. They cited the "zero tolerance" initiative that separated children from parents and a now-scrapped 2018 rule mandating that all members of a sponsors' household be fingerprinted, as well as talk of raids by ICE.

Agency statistics show in fiscal year 2018 there was a dramatic drop in the percentage of children released from ORR care compared to previous years.

Separated from his father, then labeled Category Four

Texas attorney Ricardo de Anda represents a 9-year-old former Category Four boy who was separated from his father in May 2018 as a result of "zero tolerance." They fled Guatemala after the father, an Evangelical Christian, had been "brutally attacked and tortured by members of (the 18th Street Gang) because of ... preaching against a life of crime," according to a federal court complaint filed by de Anda.

The father was sent back to Guatemala and is in hiding, but has had maintained contact with U.S. attorneys. With no other family in the U.S., the boy was classified as Category Four. During the next nine months he would be moved to four different ORR facilities, breaking his leg along the way, according to de Anda.

De Anda located a family who wanted to take the boy in, and introduced the boy's parents to them through a series of video conferences and phone calls. The father signed forms at the U.S. Embassy in Guatemala agreeing to designate the family as the boy's sponsor.

ORR rejected the request, according to court documents. "Per policy, we are not able to reunify any child with people that are not known by the family," the agency wrote to de Anda. That policy is known as the "pre-existing relationship" rule.

The boy sued and was allowed to go live with the sponsor family. ORR did not respond to questions about the boy's case.

De Anda thinks more kids should get the opportunity afforded his client. He says the government should get rid of the Category Four classification altogether.

"The reason children are stacking up in in these detention camps is because ORR does not allow qualified American families to take these children in," de Anda said. "I know for a fact, just from my practice, how many Americans are willing to open their doors to take these children in. But the door is absolutely closed to them. And as a result these children are stacking up and they're languishing."

For many kids in long-term ORR care, life can be particularly unstable, said attorney Neha Desai, Director of Immigration at the nonprofit National Center for Youth Law.

"A couple of kids I can think of off the top of my head have been in our custody literally for years, bouncing up and down (between various facilities)," Desai said.

She said kids are often beholden to the case workers tasked with vetting their sponsors.

"A case worker that is actively and creatively exploring options for a youth may be able to pursue a potential sponsor that another less zealous case manager may have never identified in the first place," Desai said.

Some Category Four children end up in ORR's foster care system, but the majority remain in ORR's network of nearly 170 shelters, some of which have been the subject of withering widespread criticism. The largest such shelter, in Homestead, Florida, was criticized in May by some of its own child residents in the form of testimonials filed in court. Children there described fear and anxiety over punishment for breaking seemingly small rules — showering too long, or hugging a sibling in violation of a no-touching policy.

An Amnesty International report released July 17 found at least 97 Category Four children in Homestead. The facility's director said that for children without sponsors, the preference is for them to be "repatriated," or deported, rather than risk that they remain in a non-relative's home where trafficking could be a risk.

"Amnesty International is alarmed by this rationale, which could result in children being unlawfully returned to harm in the countries that they fled," the report's authors wrote.

Keeping count

As attorneys, advocates and former ORR officials track similar cases, they question how the system could be both seemingly overflowing with unplaceable kids, while also apparently becoming more efficient by the day.

Agency statistics show the average length of care for migrant children dropping from a high of 93 days in November 2018 to as low as 45 days in June.

"I'm not sure how those numbers are being calculated, because it doesn't really make sense if some children are not being released to sponsors and they're staying in our care," Carey said.

He and others said they want the agency to release statistics that differentiate between the average length of care for children who have been released, and the average length of care for all children currently in ORR custody.

ORR did not reply to questions seeking average statistics that differentiate between the average length of care for children who are released, and the average length of care for the agency's entire population, including those who remain in custody.

ORR statistics show the overall population of unaccompanied migrant children has decreased from more than 13,000 in June to 10,100 as of Tuesday, amid an annual summertime decrease in immigration across the southern border. It is not clear if the number of children labeled Category Four has also decreased during that time.

"They could be indefinitely in our custody which is not a healthy situation for children, not knowing what their future is, not having access to recreation, education, being separated from their family members. And these children are for the most part already traumatized," Carey said.