The Department of Health and Human Services on Thursday asked the Pentagon to make preparations to house as many as 20,000 unaccompanied migrant children on American military bases, a United States official said.

New, makeshift detention facilities are being envisioned to house thousands of immigrant families that are crossing illegally into the United States following President Trump’s executive order on Wednesday, which called for detaining parents and children together instead of separating them.

The request comes as federal agencies on Thursday offered competing and contradictory explanations of what was happening to immigrant families in the hours after Mr. Trump’s order, leaving it unclear where families were being held and whether they were being prosecuted.