A group of private investigators and strike breakers engaged by the wealthy to quell the rabble. When you hear someone talking about it being cheaper to pay 10% of the working class to keep the other 90% down, shit like the Pinkertons is always the answer as to how.

https://guides.loc.gov/chronicling-america-pinkertons

https://daily.jstor.org/how-steelworkers-stopped-a-paramilit...

Not hard to find.