Verified answer

I would say the "main" difference is function. Your local law enforcement agencies are the ones who carry most of the load, investigating the murders, robberies, drugs and drunk driving in all our communities. State law enforcement usually has traffic enforcement on the highways and a few oddities like security of the state capitol building. Federal law enforcement takes on larger crimes with a national impact like smuggling, coordinated gang activities, and a handful of things like major kidnapping and Post Office robberies.