Washington (CNN) The Department of Health and Human Services will keep open and expand a temporary tent facility for immigrant children it is operating in Tornillo, Texas, even as most of the children it was caring for as a result of family separations at the border have been moved out of HHS custody.

According to HHS Administration for Children and Families spokesman Kenneth Wolfe, the facility will more than triple its capacity to 3,800 temporary bed spots and will stay open at least through December 31. Not all of the beds will come online at once -- 1,400 of them will be placed on reserve to be opened as needed. The facility currently has 1,200 beds.

Wolfe said family separations were "not driving" the expansion, which is being done to accommodate the high number of children who came to the US illegally by themselves. There are more than 12,800 children currently in HHS custody, a number higher than many of the weekly totals during the family separation crisis this summer. Only about 400 of those children are considered separated from families.

"HHS is legally required to provide care and shelter for all unaccompanied alien children referred by DHS, and works in close coordination with DHS on the security and safety of the children and community," Wolfe said in a statement.

It's not clear why there is still such a high number of unaccompanied immigrant children in HHS' care. Since a federal court ordered the administration to reunify all eligible separated families, more than 2,100 children have been released from custody either to their parents or other suitable adults. The Trump administration stopped separating families in order to prosecute the parents criminally on June 20.

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