HARLINGEN, Tex. — The business of housing, transporting and watching over migrant children detained along the southwest border is not a multimillion-dollar business.

It’s a billion-dollar one.

The nonprofit Southwest Key Programs has won at least $955 million in federal contracts since 2015 to run shelters and provide other services to immigrant children in federal custody. Its shelter for migrant boys at a former Walmart Supercenter in South Texas has been the focus of nationwide scrutiny, but Southwest Key is but one player in the lucrative, secretive world of the migrant-shelter business. About a dozen contractors operate more than 30 facilities in Texas alone, with numerous others contracted for about 100 shelters in 16 other states.

If there is a migrant-shelter hub in America, then it is perhaps in the four-county Rio Grande Valley region of South Texas, where about a dozen shelters occupy former stores, schools and medical centers. They are some of the region’s biggest employers, though what happens inside is often highly confidential: One group has employees sign nondisclosure agreements, more a fixture of the high-stakes corporate world than of nonprofit child-care centers.

The recent separation of some 2,300 migrant children from their families under the Trump administration’s “zero tolerance” policy on illegal border crossers has thrust this invisible industry into the spotlight in recent weeks, as images of toddlers and teenagers taken from their parents and detained behind locked doors have set off a political firestorm. President Trump’s order on Wednesday calling for migrant families to be detained together likely means millions more in contracts for private shelter operators, construction companies and defense contractors.