Washington (CNN) Federal officers have arrested dozens of undocumented immigrants who came forward to take care of undocumented immigrant children in government custody, and the Trump administration is pledging to go after more.

The news will serve as confirmation of the worst fears of immigrants and their advocates: that a recent move by President Donald Trump's administration to more fully vet people who come forward to care for undocumented immigrant children who are alone in the US has been a way for the administration to track down and arrest more undocumented immigrants.

On Tuesday, Immigration and Customs Enforcement senior official Matthew Albence testified to Congress that, after Health and Human Services and ICE signed a memorandum of agreement to background-check and fingerprint potential "sponsors" of immigrant children, ICE arrested 41 people who came forward.

In response to an inquiry from CNN, an ICE official confirmed that 70% of those arrests were for straightforward immigration violations -- meaning they were arrested because ICE discovered they were here illegally.

The individuals could have been the children's parents or family members, and they also could have merely been fellow members of the homes of adults who applied to care for the children as they fight for a legal right to stay in the US.

"We've arrested 41 individuals thus far that we've identified pursuant to that (memorandum)," Albence testified Tuesday. "Close to 80% of the individuals that are either sponsors or household members of sponsors are here in the country illegally, and a large chunk of those are criminal aliens. So we are continuing to pursue those individuals."

ICE made those arrests from early July through early September; of those, only 12 were criminal arrests, according to an ICE official speaking on condition their name not be used.

The remaining 29 were what are known as non-criminal or "administrative" arrests -- as in immigration violations.

The memorandum of agreement was signed in response to concern about past instances of children being placed in dangerous situations and HHS' inability to account for the whereabouts of hundreds of children shortly after they were released. But experts and advocates have feared the agreement to background-check and fingerprint through ICE would scare off potential sponsors.

The number of immigrant children in HHS custody has been skyrocketing to record levels, with more than 13,000 in custody as of Thursday. The average length of stay in custody has nearly doubled since 2016, to an average of 59 days, and the rate of release has dropped by thousands of children each month. The crowding has prompted HHS to triple the size of a temporary tent facility it opened in Texas at the height of the family separations crisis.

Experts and an official with knowledge have pointed to the memorandum as the likely reason for the growth since, in addition to other policies that have kept children in custody longer.

It has long been true that undocumented children who arrive in the US by themselves will often seek to be placed with relatives who may also be living in the US illegally. The administration has described this process as a circumvention of law in order to exploit more lenient policies for children, even labeling it "smuggling" of children.

But the advocates who work with the children and the attorneys who represent them say many of these children are fleeing extremely dangerous situations in their home countries and have legitimate claims to stay in the US that could take years to pursue. Family and friends already in the US can provide stable homes for them as they pursue those avenues of legal status.

HHS does not consider a person's immigration status as a factor in whether they are suitable to care for children, and the Obama administration did not make a practice of arresting people who came forward as sponsors. The Trump administration has hinted it would take the opposite approach, and Thursday's revelation confirmed that.

The administration has made clear it considers any undocumented immigrant to be eligible for arrest, and has targeted laws designed to protect children as "loopholes" that attract more illegal immigration.

Experts who focus on children warn that using the HHS system -- which is designed as a bridge for traumatized kids into a situation where they can make legal claims to stay in the US -- as an arm of law enforcement is contrary to the principles of child welfare.

Meanwhile, the system is already taxed beyond its normal population.

HHS' Office of Refugee Resettlement "has a hard enough job to do when they can be focused on being a family-serving organization charged with reuniting children with their family, and when that mission is compromised by making them collect information for the purposes of immigration enforcement, that runs contrary to their primary mission and it's contrary to the best interests of children," said Maria Cancian, a former HHS deputy assistant secretary and a professor at the University of Wisconsin-Madison.

A former Obama administration official worried the government's new tactics could harm children in its custody.

"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 at the end of the Obama administration.

"The question I would ask is, are measures legitimately enhancing the security situation?" added 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?"

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.

"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."

CLARIFICATION: This story and its headline have been updated to clarify who ICE has arrested.