Many countries have national polices. Czechia has the Police of the Czech Republic, which I see in Prague more often than Prague police. Canada has the Royal Canadian Mounted Police, whose presence is not really felt in Toronto, but their role is by no means insignificant. However, there doesn´t seem to be one official national police in America. You have the FBI for intelligence and a number of other law enforcement agencies under different departments. But there is one whose name crops up from time to time, and these look to me like the closest you have to a national police - the U.S. Marshals Service.
Or is that their role? They seem to keep a very low profile. What exactly is their mandate?