Detailed article from WaPo, summarized at BoingBoing:
The Washington Post tracked down workers from Santa Teresa de Cajon in Costa Rica, who say that they and their neighbors were part of a "pipeline" from Central America to Trump properties in New Jersey and elsewhere, where they worked doing construction, groundskeeping, and cleaning, with the full knowledge of their supervisors and Trump Organization managers (a claim verified by a police report detailing a warning from local officials to Trump Organization bosses about the number of undocumented workers on Trump's property).
The workers say they were paid about 20% of the local wage for their labor, and that their undocumented status was weaponized against them: Trump managers refused basics like raincoats for outdoor workers during storms, set lawnmowers speeds so high that workers would have to run to keep up with them for hours on end, and, of course, denied them routine workplace necessities like health care, sick days, etc. They describe being verbally abused and threatened by their managers.
The Post also tracks down reports and first-hand accounts of other workers from South and Central America who worked illegally at Trump's properties, who claim that Trump's Bedminster, NJ golf course, the so-called "Summer White House," was built by undocumented immigrants.
Trump and his family have repeatedly stressed that they personally oversee every detail of the construction, outfitting and operation of the businesses that bear their name. In 2011 (shortly after the completion of the Bedminster project) Eric Trump appeared in a promotional video where he denied that Trump was just the face of the operation, continuing "We design every single tee, every fairway. . . . We pick the carpets. We pick the chandeliers. There is not one element of these clubhouses which we don’t know about it. You name it — we’re involved."