US has quite a few national agencies. Most prominent among them are Federal Bureau of Investigation or FBI, Alcohol, Tobacco, and Firearms or ATF, Immigration and Customs Enforcement or ICE, Drug Enforcement Administration or DEA, and Department of Homeland Security or DHS. Each deals with a specific area of expertise.
Most agencies in states are broken up by municipal level, which is your cities and towns. County level which is in some states the agency that deals with rural crime and areas which no municipal force is present, in other states, they are the most powerful agencies. Then state level, which functions just like county, sometimes the rural crime enforcers and "highway police" and sometimes the highest tiered agency. All depends on geography and every state is organized differently.