McALLEN, Texas — A defense official says the Pentagon has agreed to provide housing on military bases for up to 20,000 migrant children detained after illegally crossing the U.S.-Mexico border.

The official says the Department Health and Human Services asked for the space and the Pentagon agreed to support it. The official had knowledge of the request and spoke on condition of anonymity because the arrangement has not yet been announced.

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It was first reported by The Washington Post.

It’s unclear which military bases would be used to house the children. HHS has assessed four military bases as prospective housing for child migrants.

Three are in Texas: Fort Bliss, Goodfellow Air Force Base and Dyess Air Force Base. The fourth is Little Rock Air Force Base in Arkansas. It’s not clear whether other bases are under consideration.

The prospect of the U.S. government housing up to 20,000 migrant children on military bases has raised questions on Capitol Hill.

Sen. Chuck Schumer said Thursday that he wants to know how many children are already being held after illegally crossing the U.S.-Mexico border and in what conditions.

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Schumer asked why reporters haven’t been allowed to tour detention facilities and how the new plan would work. The New York Democrat also wondered how the Trump administration was keeping track of families separated under its “zero tolerance” policy on illegal border crossings.