Modern slavery in the US often works by bringing people in from another country, requiring them to pay for lodging and food at a "company store," and paying low enough wages that their debt steadily increases. Then you hold their passports and tell them they can't leave until they're paid up.

It's illegal, of course, because it's literally slavery despite the window-dressing of money changing hands. But the slaves are vulnerable and don't necessarily know it's illegal, or they're threatened.