Law Enforcement Agencies of the United States of America
Note: this information is collected from various sources, and its accuracy is not guaranteed due to the variegated nature of the subject matter and differences in code from place to place. Consult a lawyer if the answer to your question is important to you. Tell me if it's incorrect.
The U.S. is big. Geographically big. It has a large population, who live in all manner of regions ranging from absolute wasteland to large cities. Due to the size of the country, the evolving complexity of its legal code, and the accidents of history, the U.S. has a plethora of various agencies tasked with law enforcement.
The Bottom-Up View
This is the best way to get a glimpse of the various powers of arrest, detention, deportation, regulation, and the like that weigh down on you, a citizen or visitor standing in the U.S. Let's start with where you're standing, and work 'outwards and upwards' to discover the various law enforcement organizations that might have jurisdiction over you and your actions.
- Private Police Forces. These are not the same as rent-a-cops! It is possible for private institutions to have their security personnel accorded the status of police if they are trained at state-approved facilities and the state approves the application. Typically, these police forces are limited to institutions and organizations that maintain physical grounds, and have authority (power of arrest, etc.) only on those grounds. For example, large universities in the U.S. sometimes maintain their own police. Let's say you are standing on a university campus; their police force has authority.
- Town/Borough/City/Village/etc. (Municipal) Police. The municipality that contains this institution (if there is one) will likely have its own police force. These are professional police officers; that is, they are employees of the municipality, and their actions are subject to the regulation of the U.S. State in which they are employed. They are not elected. Sometimes their chief may be. In the event of a more serious crime taking place inside that university you're standing in, they will be called in - usually any crime requiring additional investigation, as the University police won't have the personnel or facilities.
- County Sheriff. If you are within a U.S. County but outside the boundaries of any particular municipality with its own police force, you are likely in the jurisdiction of the Local Sheriff. Sheriffs are typically elected officials of the county; whether they themselves have formal law enforcement training will vary from place to place. They will usually have deputies, however, who are professional police officers, and have been trained. Their departments will sometimes contain resources (labs, records, etc.) that are used to handle incidents outside the jurisdiction of a municipal police force, or which can be called in to assist that police force in its work.
- State Police. Each U.S. State has an official policing organization. Usually, the State Police are concerned with law enforcement on the federal and state highways, although some states (such as California) have a separate or subordinate organization dedicated to highway patrol. They also serve as the 'local' law enforcement in remote areas of a state not covered by a county or municipality police force. Their resources are sometimes available to lower-level police as well. Sometimes called 'Staties' or 'State Troopers', they are a professional police organization, usually commanded at the state level by a Gubernatorial appointee or professional commander.
Believe it or not, that's the end of the line, sort of. There are no overarching 'default' jurisdictions above that of the states - we're a federation of states, remember? Actually, in practice, this isn't quite true, and there are several agencies which, at the national level, carry out various forms of law enforcement. Here are a few of them.
- The Federal Bureau of Investigation (FBI). The Feds. G-Men. J. Edgar Hoover's boys and girls. The FBI was created to undertake criminal investigations and pursuits that, by their very nature, required coordination and action across state borders. Exactly how and why it came to be is worth the several books that cover it, but let me gloss over all of that by noting that it was created during a time of now-legendary crime figures whose operations (empires) and exploits ranged across the U.S. and occasionally beyond. The F.B.I. was the U.S. Government's means of applying resources to law enforcement. It became tasked, eventually, with additional jobs of a national scope, most famously that of counterintelligence, since the organization was already chartered to enforce the law within the borders of the U.S. F.B.I. agents are agents of the Justice Department. To quote their publications:
The Federal Bureau of Investigation (FBI) is the investigative arm of the US Department of Justice. The FBI’s investigative authority can be found in Title 28, Section 533 of the US Code. Additionally, there are other statutes, such as the Congressional Assassination, Kidnapping, and Assault Act (Title 18, US Code, Section 351), which give the FBI responsibility to investigate specific crimes.
- The Bureau of Alcohol, Tobacco and Firearms (BATF). Also known as T-Men. Members of this agency in fact used to be employees and agents not of the Justice Department, but of the Treasury, which had typically been empowered to prevent and interrupt the smuggling of these controlled cargoes across state lines. On Jan. 24, 2003, however, the ATF was transferred to the Department of Justice, joining their FBI colleagues. The ATF is called in to deal with major violations of federal regulations concerning the production, sale, distribution, storage or use of (again) alcohol, tobacco or firearms - all heavily regulated industries. In the event of the BATF becoming involved, local law enforcement will typically be asked/assigned/ordered to support the ATF's actions - since the crime in question is a federal crime.
- The Drug Enforcement Agency (DEA). The DEA is yet another bureau of the DOJ chartered to handle any violations of drug-related federal law - smuggling, production, distribution, etc. They operate similarly to the BATF, being tasked to handle incidents or investigations dealing with federal-level drug-related crimes and enforcement of civil regulations arising from the Controlled Substances Act.
- The Secret Service used to be part of the Treasury, since their initial mission was to handle threats against the U.S. financial and monetary system such as fraud and counterfeiting. They still handle matters related to these areas, but also have taken responsibility for the protection of important government and diplomatic personnel such as the President of the United States and his family; the Vice President, Presidents-elect, former presidents, foreign dignitaries, etc. etc. They have a uniformed police division which handles routine security for diplomatic posts and White House areas in addition to the plainclothes agents on protection details. In 2003, the Secret Service was also transferred to the Department of Homeland Security from the Treasury. Trivia: the FBI was formed around eight agents initially transferred from the Secret Service to the Department of Justice. The Secret Service is responsible for investigating threats to the President and their other protectees at a policing level as well as providing 'shield' protection.
- The Armed Forces have their own investigative and policing branches who are responsible for law and order on those lands within (and without) the United States under military jurisdiction - military bases, ranges, military reservations, etc. etc.
- muted has reminded me that there are various railroad police which are federally chartered to patrol the trackage and rolling stock of the railroads, which were originally federal land grants. As one example, Amtrak police handle policing onboard Amtrak trains and on Amtrak railways (there aren't that many of those; Amtrak rides mostly on other companies' rails).
Those are the biggies. There are numerous smaller, more specific policing organizations. The Postal Inspection Service which handles mail fraud, identity theft and other mail-related crimes, for example; or the Capitol Police, charged with policing the grounds of the U.S. Capitol in Washington D.C. Federal Air Marshals are officially FAA employees (I think) who police aircraft in flight. Moving astray from 'policing' - there are Federal Marshals, whose job is familiar to anyone who has seen The Fugitive - they find and retrieve fugitives, as well as handle enforcement actions related to manhunts. The Immigration and Naturalization Service (INS) had its own agents, as well as calling in assistance from other agencies when needed; it's been split into three Bureaux by the Department of Homeland Security.
Note that all of the above organizations and persons can and do work together. There are byzantine and exhaustive rules about which are supposed to have authority and/or jurisdiction in any particular case, but like any other set of rules governing the behavior of organizations, they often give way to politics. As anyone who has watched an American cop movie can attest, jurisdictional frictions are a way of life - although in many areas of enforcement, such cooperation occurs frequently enough (or continuously) that the organizations work smoothly and in concert.
There is no single 'system,' as well. Each state has different laws and regulations that govern the operations and behavior of law enforcement personnel and organizations. Informal agreements exist to cover 'grey areas.' Tacit arrangements have been worked out. Also, note that even if a law enforcement officer is outside his or her jurisdiction, if they witness a crime they can always detain suspects and place them under citizen's arrest, or just hold them for local law enforcement. Most times, uniformed officers who undertake such actions will simply hand off the paperwork and legal authority to the local law enforcement agents, with their thanks. Be careful - as a private citizen, your word isn't as good as theirs (a fact of life) and you're not trained to deal wth these things. Performing a citizen's arrest is an act which carries severe consequences, even if you're right - the best way to handle any situation you might witness is to find help fast. If you can't find help, then it's on your head to decide what to do.
It has been brought to my attention that there are some well-deserved points of confusion about the command of said law enforcement agencies, especially from non-U.S.ians. That's not surprising; most of us have no clue what's going on. I'm going to attempt to dazzle you with glittery phrasing and sprinkles of information whilst hiding that fact that, yes, I have no idea what I'm talking about and that I swear I am not making this up.
There are two types of commanders for policing forces in the U.S. - hired and elected. In some places, the head of the local law enforcement is an elected office. This is most often true in the case of the county Sheriff. I believe that in some places, the Police Chief is an elected official. The real problem is that there are a dizzying number of these positions across the U.S., each with its own rules and traditions.
In general, however, a good rule of thumb is this. Anyone with 'police' in their title has been hired or appointed by a politician - this is a safe bet. Sheriffs are typically elected, while their deputies are not. At the state and national level, law enforcement agents tend to be hired or appointed; while their civilian bosses can give them orders (Governors, Mayors, Presidents) those bosses are not actually members of the force; we're just exercising our penchant for civilian control of armed force, here. At the federal level, all law enforcement agents are hires or appointees.