(NewsNation) —  Nearly 20,000 migrant children who came over the southern border without adults are unaccounted for by the Biden administration.

According to the Department of Health and Human Services, the federal agency has been unable to get in touch with the more than 19,000 sponsors to whom it had released these unaccompanied minors.

The department said these children are typically given to family members or people who have been vetted while they pursue their cases in heavily backlogged immigration courts.

But staying in touch with them has been a persistent issue for Health and Human Services.

Almost one in three calls to migrant children or their sponsors went unanswered between January and May, Axios found. However, during that same time period, Health and Human Services discharged 32,000 children and teens, according to the news outlet, but placed fewer than 15,000 follow-up calls. Data from the department shows that the number of children discharged was twice as high as the number of calls checking in on them in the following month.

These calls are made to determine whether the migrant child is still living with their sponsor, is attending school and is aware of their upcoming immigration court dates, Health and Human Services said on its website.

“While we make every effort to voluntarily check on children after we unite them with parents or sponsors and offer certain post-unification services, we no longer have legal oversight once they leave our custody,” a Health and Human Services spokesperson said to Axios. The spokesperson added that many sponsors do not return phone calls or don’t want to be contacted.

The Trump administration, Axios noted in its report, was also criticized for being unable to account for around 1,500 children released from shelters run by Health and Human Services during a three-month period.

An increase in children traveling alone has severely strained border holding facilities, which aren’t allowed to hold people for more than three days, leaving the government scrambling to find space and staff.

The Associated Press contributed to this report.