I'm in the middle of reading Colson Whitehead's The Underground Railroad, which I realize is fiction, and fantasy/magical realism at that. In the book, the United States government has gone into the slave owning business, having accepted human property in the form of "inventory" liquidated from plantation bankruptcies. The individuals in question, who are still enslaved though given a degree of personal freedom (a work week, days off, access to education, etc), are hired out as workers, paid a wage, and live in much the same conditions as a free urban industrial worker of the time might. Now, having read some antebellum history, this seems like it's probably one of the more heavily fantasy-based elements of the story. As a reader, too, it seems clear to me that this part of the story is a metaphor for the larger themes of the nature of slavery vs. freedom and not meant to describe specific historical events.
But it got me thinking. The White House, Capitol Building (the building of which is referenced in the novel, even), and other major federal building projects were (apparently?) undertaken using slave labor. How exactly did this work? Did the federal government contract it out? If so, with whom? How was it administered? What were working conditions like? Beyond things like Michelle Obama's famous quote "I live in a house built by slaves," is it widely known exactly which government projects used slave labor, and in what capacity?
I actually answered almost this exact same question a year ago . You can check out my answer here. But before reading it, I’d like to preface that your question makes the assumption that the government would have to own slaves in order for them to work on government buildings. This is incorrect. As in the case of the White House, which did have enslaved workers, they were not owned by the government, but rather owned by the folks who were hired to build the buildings. The White House was not the only building built by slaves in Washington D.C. The Capital building was partially built by slaves (it’s construction took place prior to, during, and after emancipation). It is likely that other buildings were built by enslaved people since both Virginia and Maryland that surround it (where companies and workers would have been hired from) were deep slave territories.
I hope this helps. Please feel free to ask follow ups if I didn’t fully answer your question.