In many cases the feds are unmarked. Most I have seen that are marked as FED Police are contractors. In other words say the fellows that guard the nuclear plants, dams, missile sites and federal buildings. They are granted federal arrest powers, but are usually not fed employees. Pinkerton, Wakenhut or one of the other major security firms usually employ them outright or get an eskimo tribe or other minority orginization to employ them on paper. Most of the more obscure fed agencies will put what they are, such as TVA Police, US Parks Police, Library of Congress Police and so on, instead of fed police. Some of them have changed from police to protective services now to be less intrusive sounding. There are most likely a hundred or more fed policing agencies that we don't even know about.