Washington (CNN) A record number of immigrant children are in US custody, and it's likely because the Trump administration's policies are keeping them there.

As of this week, there are 12,800 immigrant children being cared for by the Health and Human Services Department. That's the most ever, an HHS spokeswoman confirmedIn 2016, the monthly average of the number of children in care ranged from just over 4,000 to over 9,000.

But that figure isn't the one that's raising eyebrows among experts. It's seen as a symptom of a bigger issue.

According to an official with knowledge not cleared to speak publicly, the rate of children being released from HHS has plummeted substantially. At the same time, the average length of time children stay in custody is skyrocketing.

The cause is likely moves by the Trump administration in its aggressive efforts to tighten immigration. Experts fear the result will transform a system created and designed to help put traumatized children on a path to stability into a way to punish them and send a message.

"These are kids who fled some of the most violent countries in the world, many have experienced trauma ... rape, robbery, all kinds of exploitation," said Bob Carey, who ran the HHS office overseeing child detention from 2015 to early 2017.

The administration moved last spring to better scrutinize adults coming forward to take care of the children as they pursue a right to stay in the US. That agreement includes exhaustive screening conducted by Immigration and Customs Enforcement. But many of the adults who take care of these kids are undocumented themselves, and fearful of ICE getting their information.

Additionally, lawsuits have accused the administration of other moves that have extended children's stays, including allegedly holding them intentionally until they turn 18 and eligible for stricter adult detention. Many of the children do have legitimate legal rights to stay in the US, but the legal process to gain them can take years.

"The question I would ask is, are measures legitimately enhancing the security situation?" said Carey, who's now a leadership and government fellow with the Open Society Foundations. "The ultimate security is not releasing any child to a sponsor, because then nothing would happen to them. But how much harm are you causing by keeping kids in custody indefinitely in settings that were never designed for that?"

According to the official with knowledge, the daily discharge rate for unaccompanied immigrant children is down to 0.6 per hundred, from 2.0 per hundred in 2017. With a population of 12,800 children, over a 30-day month that would translate to only 2,304 released, vs. 7,680 at the old rate. That means more than 5,000 more children kept in custody per month.

Likewise, the average length of stay in custody is rapidly climbing. According to annual reports to Congress , in fiscal year 2016, the average length of stay had been brought down to 35 days by the Obama administration. The average in 2017 went up to 48 days.

Today, that average is up to 59 days, according to HHS spokesman Kenneth Wolfe. And that counts only children who have already left, experts point out, making it impossible to know how long some kids have been waiting.

The system was further taxed this summer by a decision to separate immigrant families at the border in order to criminally charge adult parents who entered the US illegally. The children were declared unaccompanied and shipped off to HHS facilities like children who arrive illegally by themselves. More than 2,600 separated children were turned over before the separations were halted -- 400 of whom remain. In June, nearly 11,900 children were in HHS care.

HHS spokeswoman Evelyn Stauffer said the administration is enforcing the law to address a "crisis at the border."

"The number of families and unaccompanied alien children apprehended are a symptom of the larger problem, namely a broken immigration system," Stauffer said in a statement. "Their ages and the hazardous journey they take make unaccompanied alien children vulnerable to human trafficking, exploitation and abuse. That is why HHS joins the President in calling on Congress to reform this broken system."

But the number of children crossing the border alone has been consistent with rates the last four years. In August, roughly 5,000 children were caught trying to enter the US alone illegally or without permission at the southern border.

With the plummeting discharge rate, however, the number of children in custody could skyrocket even at steady crossing rates. HHS announced Tuesday that it will triple the size of an emergency tent shelter for children in Texas.

Experts fear the result will be traumatized children held longer in a more crowded and overwhelmed system, putting them further at risk.

Trump policy changes

The current official, Carey and another former HHS official all pointed to the administration's heavier vetting procedures as a primary cause, compounded by the overall chill on immigrants under this administration.

The Trump administration last spring announced the ICE-HHS partnership to more heavily scrutinize adults who come forward, including fingerprinting. But the former officials said there were already plenty of ways to screen for things like criminal records and history of abuse without including ICE and immigration status.

In September 2017, then-ICE acting Director Tom Homan said at a public event that his agency would arrest undocumented people who came forward to care for the children, something previous administrations avoided.

"You cannot hide in the shadows," Homan said at a Washington border security event, adding that parents should be "shoulder-to-shoulder" with their children in court. "We're going to put the parents in proceedings, immigration proceedings, at a minimum. ... Is that cruel? I don't think so."

But child welfare, like medicine, for example, has a fundamentally different mission than immigration enforcement, said former HHS Deputy Assistant Secretary Maria Cancian, a professor at the University of Wisconsin-Madison. Muddying the two could undermine the best interests of the children, she said.

Not only does the new vetting policy frighten potential sponsors, she said, but it also could push some to seek a stranger to pose as the kid's sponsor instead, adding even more risk to the system. Any policy should be weighed for its pros and cons, she said.

"In the child welfare system, we worry about reuniting kids with families before we've dotted every I and crossed every T, and we worry about the trauma of keeping kids in the foster system," Cancian said. "I would argue that sharing information on the immigration status of sponsors with ICE, I don't see how that would have any impact on child safety (vs.) the chilling effect it would have."

There are indications that the Trump administration may also be slowing kids' release in quieter ways.

In June, a federal judge blocked current program Director Scott Lloyd from continuing to require his personal sign-off on the release of a child from a secure facility, calling it "the zenith of impermissible agency action" and an "unconscionable delay" for kids.

"Vulnerable (immigrant children) cannot and should not be held hostage to an administration's flight of whimsy," wrote District Judge Paul A. Crotty.

Last month, another federal judge granted class action status and let proceed a lawsuit that alleges the administration was improperly keeping teens in custody until they turned 18 so they could be transferred to more restrictive, adult detention.

The three immigrants bringing the lawsuit all had similar stories of being held. One boy started working at 6 years old and came to the US at age 17. The day before his 18th birthday, his attorney asked that he be released. He was transferred to adult detention.

Another girl said she was fleeing longtime abuse back home. Numerous potential sponsors for her backed out under the government's tough screening process. Her attorneys say her post-traumatic stress disorder was exacerbated in detention.

A third teen had a baby at 15 years old and brought her child with her to the US. The court filing says that when she turned 18, she was separated from her daughter, who remained in HHS custody, and was transferred to adult detention.

"You can make anything absolutely secure, you can stop any tourists from coming into the US, you can stop any travel, but what's the cost of that?" Carey said. "That can also destroy your economy in the process or violate the basic laws and tenets on which your nation has been built."