There were only two other departments and they are always used as extra bodies and invited as a courtesy to local LEOs.
Really? Always? From whence does this assertion stem?
And why not the FDA? Forestry? SEC? etc? They all have police powers too, don't they?
I shouldn't admit this especially here. I was an MP in the military. When I got out I got my post certification and worked as a LEO for a while. County sheriffs are invited to all major busts because you don't want to step on toes and sometimes you need local booking and lockup facilities. The Marshal service is everybody's bitch to use if you need them. lol