President Donald Trump said during a Monday morning interview with Fox News that the federal government will "build tent cities" for thousands of asylum-seeking migrants slowly making their way north toward the U.S. border in two large caravans from southern Mexico and Central America.

Mr. Trump said in the interview with Laura Ingraham that the administration will detain any Central Americans who complete the more than thousand-mile trek while they await "their trial" after claiming asylum. The group is still weeks or more away from reaching the U.S. border.

"We're going to build tent cities. We're going to put tents up all over the place. We're not going to build structures and spend all of this, you know, hundreds of millions of dollars — we're going to have tents," Mr. Trump said.

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But tent cities are not a cheap solution. Hundreds of millions of dollars is how much the one tent city currently operated by the federal government is costing this quarter alone. The U.S. Department of Health and Human Services (HHS) said in a Sep. 18 announcement in the Federal Register that it expected to spend $367 million on the government's tent city at the Tornillo Port of Entry in Texas in just the final three months of the year.

That facility currently houses about 1,500 unaccompanied migrant minors, but was recently expanded to make room for up to 3,800 beds.

By comparison, the two large migrant caravans traveling through Central America and Mexico are estimated to include a combined 6,500 people.

While the tents at Tornillo are located on federal land, documents obtained by CBS News through a Freedom of Information Act request show HHS has struggled to find new locations to house migrants, in particular sites for tents.

On August 2, Capt. Gregory Davis, HHS' Director of the Division of Unaccompanied Alien Children Planning and Logistics, wrote in a letter to other HHS personnel that the search for potential new migrant housing sites that were "land only," and had no previously constructed buildings, was "becoming problematic for us."

Instead, the agency was seeking non-tent "hard sided facilities in both the private and federal sectors," preferably in areas of central Texas "largely away from hurricane and flood risk."

The search for such a facility, Davis wrote, could include, "Looking at a Texas map, anywhere in a circle roughly outlined by Lubbock, Midland, San Antonio, Austin and Dallas. I know that is a massive section of TX."

An attachment to Davis' email, included below, describes the types of properties that could be used for an unaccompanied minor facility.

Questions sent to HHS about "tent city" locations and budgets were not immediately returned.