A special agent, in the United States, is usually a criminal investigator or detective for a federal, state, or county government who primarily serves in investigatory roles. Within the US federal law enforcement system, dozens of federal agencies employ federal law enforcement officers, each with different criteria pertaining to the use of the titles special agent and agent.
In intelligence usage, agent refers to one who is recruited, trained, controlled, and employed to obtain and report information.
In general, only some agents are federal law enforcement officers and hold either arrest authority or the right to conduct minor criminal/non-criminal investigations. In some agencies, however, an agent may have both arrest and minor criminal/non-criminal investigatory authority but still have no authority to conduct major criminal investigations.
On the other hand, nearly all special agents are federal law enforcement officers, are distinctly empowered to conduct both major and minor criminal investigations, and hold arrest authority. Most special agents are authorized to carry firearms both on and off duty, but only some agents are authorized to do so. In US federal law enforcement, the title of special agent is used almost exclusively for federal criminal investigators.
Within the U.S. government, the title of special agent designates a general investigator in GS-1810 job series, federal criminal investigator in the GS-1811, Diplomatic Security Service FS-2501 job series, or a counterintelligence operations specialist in the GS-0132 job series according to the Office of Personnel Management (OPM) handbook. Special Agents typically have an undergraduate degree. They are usually empowered to carry firearms, make arrests, and investigate violations of federal laws. The titles of "Agent", "Investigator", and "Background Investigator" refer to all other 1800 series job series designations, including:
Although some federal agencies have recently defined certain background investigators and other non-1811 personnel as "special agents", this is purely an administrative title with a different meaning, and does not confer actual Special Agent status on them. Although some 1810-series investigators are empowered in some circumstances to carry firearms and investigate crimes, they are not empowered to make arrests. In general, arrest authority is what distinguishes a traditional Special Agent (1811) from most other 1800 series personnel. There are exceptions: some 1800 series positions, such as Border Patrol Agents, Federal Air Marshals and Secret Service Uniformed Division Officers have the authority to enforce federal law, and thus have arrest authority. However, unlike the 1811 Criminal Investigators, or Special Agents, they do not have the authority to conduct major criminal investigations. In general, non-1811 personnel who wear a badge and a gun and have arrest authority are considered "Agents", while Criminal Investigators who carry a badge, gun, and credentials and have arrest authority are considered "Special Agents". All other 1800 personnel who conduct investigations (background or otherwise) and who do not have arrest authority are "Investigators", "Background Investigators", or "Compliance Officers", no matter what their administrative title might be.
Most federal agencies, including the following, employ some type of special agent, investigator or background investigator:
Federal law enforcement training can be divided into various categories, the most common being basic, agency-specific basic (ASB), advanced/specialized, and agency-advanced/specialized. To operate safely and effectively, U.S. special agents and criminal investigators must possess skills and knowledge regarding criminal and civil law and procedure, enforcement operations, physical techniques, and technical equipment, to mention a few. They must also be physically fit. While possession of a college degree can aid in obtaining employment in this profession, only extensive training provided at specialized facilities, combined with on-the-job training, can provide the skills and knowledge needed to perform the duties of a federal criminal investigator. As of 2012, there were 13,913 FBI agents, and as of 2011, there were 4,890 DEA agents in the United States.
U.S. special agents and federal criminal investigators generally receive their basic training at one of two primary locations: the Federal Law Enforcement Training Centers (FLETC), with facilities in Brunswick (Glynco), GA, Artesia, NM, Charleston, SC, and Cheltenham, MD; or the FBI or DEA training facilities based at the FBI Academy in Quantico, Virginia.
Special Agents of the FBI and DEA may also receive additional training focused on working in conjunction with the US Military overseas. During the War on Terror many Special Agents deployed in support of counter-terrorism operations.  
Only DEA and FBI agents receive their basic training at Quantico. The FBI and DEA operate completely self-contained academies that provide all levels of training to their agents. These academies make no distinction between basic and agency-specific basic training. New FBI and DEA agents train at their academies for approximately five months before they begin their first investigative assignment. Both agencies' academies also provide advanced training in various subjects to other federal, state, and local law enforcement agencies.
The FLETC, commonly pronounced "flet-see", is a consolidated training facility that provides basic and advanced training to U.S. special agents and other federal law enforcement personnel not employed by the DEA or U.S. Postal Inspector Service (USPIS). The FLETC also provides advanced and specialized training for most federal, state, local, and non-U.S. law enforcement agencies willing to share in the cost. The FLETC's basic training course for special agents, the Criminal Investigator Training Program (CITP), lasts about 13 weeks, and may be given to persons if required for self-protection, depending on changes to program content or holidays. Nevertheless, CITP only represents the beginning or basic training received by U.S. special agents not employed by the FBI, Federal Air Marshals, DEA, or USPIS.
After completing the FLETC CITP, most agents immediately transition to training provided by their own agencies (hence the term agency-specific basic, or ASB), lasting another 2 to 16 weeks and sometimes longer, depending on the agency. Some smaller agencies, like the 64 Offices of Inspector General (OIGs), operate consolidated academies, such as the Inspector General Criminal Investigator Academy (IGCIA), through which specialized but common ASB-type skills and knowledge are more economically taught. So agents employed by the OIGs first attend CITP, then attend the IGCIA's IG Investigator Training Program (IGITP), then attend their own agencies' ASB training after completing IGITP, receiving a total of up to 16 weeks or more of training before conducting their first investigation. Many of the agencies utilizing FLETC maintain their individual academies for providing ASB and agency-specific advanced training on the same grounds as FLETC and share use of the same facilities. Some agencies, such as the U.S. Department of State's Diplomatic Security Service and the U.S. Secret Service, conduct their ASB training in separate agency-owned and operated facilities.
Although a much smaller agency than the FBI or DEA, the U.S. Postal Inspection Service also operates a self-contained federal law enforcement training academy called the Career Development Division (CDD). Like FLETC and Quantico, USPIS CDD has been fully accredited by the Federal Law Enforcement Training Accreditation (FLETA). Like the other academies, CDD provides basic training to postal inspectors in firearms, legal use of force, driving training, crime scene management, controlled deliveries, felony arrests, case management, case development, informant management, and surveillance, but also incorporates agency-specific basic training to help prepare the USPIS candidates in enforcing postal laws and federal mail statutes such as mail fraud, mail theft, and other mail related crimes. In addition to basic training, CDD also provides advanced training for the postal inspectors, the uniformed postal police personnel, and the analysts.
For all U.S. special agents, training does not stop after basic and ASB. The career of a federal special agent is one of regular training in new legal issues and investigative techniques, and frequently includes quarterly, if not monthly, refresher training in hand-to-hand defensive tactics, the use of weapons of less than lethal force, and regular qualification in the use of firearms.
Not all federal criminal investigators are called special agents. Some federal agencies entitle their investigators as criminal investigators but use the term interchangeably with special agent. Other federal agencies use different titles for the same 1811 criminal investigative job series. Series 1811 criminal investigators for the U.S. marshals are entitled deputy marshals. Series 1811 criminal investigators for the U.S Postal Inspection Service are called postal inspectors. These inspectors were originally called surveyors and received a title change in 1801 to special agent. In 1880, the U.S. Congress created the position of Chief Postal Inspector and renamed these special agents to postal inspectors. The first special agents in the United States were appointed in 1791 when the Secretary of the Treasury was authorized to employ "Special Agents" for the purpose of examining the accounts and books of the Collectors of Customs. The position of Special Treasury Agent was created, and until 1860 submitted reports to the Department of Treasury, through the Collectors of Customs in the Customs District in which they were employed.
The titles special agent and secret agent are not synonymous. The title special agent is commonly the official title assigned to individuals employed in that capacity, especially by the U.S. agencies described above (and for the reasons described below), whereas secret agent is less of an official title, but is used to describe individuals employed or engaged in espionage. Special agents, like state, county, and municipal law enforcement officers, can, at various times, engage in secret or undercover activities as part of investigative operations, or counter-espionage assignments, during which they might be referred to as undercover agents. They may also be referred to, or refer to themselves, as criminal investigators, federal agents, U.S. Agents, U.S Special Agents, U.S. Federal Agents, agents, federal authorities, federal officers, federal investigators, or U.S. officers.
Criminal Investigators of the U.S. Secret Service conduct criminal investigations pertaining to the homeland security department as well as financial and electronic crimes, and are "Special Agents". But Secret Service Special Officers who assist Special Agents on protective details only and do not conduct criminal investigations of any kind are, like the USSS Uniformed Division Officers, only considered to be "Agents".
At the management level, the head of a region or office might be called a special agent in charge, abbreviated as an SAC or SAIC. The Deputy Special Agent in Charge (DSAC), acts as the operational manager for investigations and typically supervises Assistant Special Agents in Charge (ASAC's). First line supervisors may be called simply Supervisory Special Agent. Assistant Special Agent in Charge, abbreviated as ASAC, are second line supervisors.
Exactly which U.S. special agents have the broadest authority is a matter of debate. The issue of concurrent jurisdiction (in which multiple agencies have non-exclusive jurisdiction over a given set of the U.S. Code, such as the FBI, HSI and DEA in respect to drug laws) does not make the issue clearer. However, special agents of many agencies can often enforce Title 18 (general criminal code) of the US Code included in their agency specific duties. Some U.S. special agents have a larger number of Titles, or just different Titles, to enforce and with those titles sometimes include unique authorities.
The following are some examples of unique authorities that agency-specific special agents possess: Administrative subpoena powers. Department of Defense and any military special agents are authorized to enforce the UCMJ. Title 19 designation authorizes CBP officers, Border Patrol Agents and HSI special agents to perform border searches, which require no suspicion or warrant.
Some agencies have been designated lead agency by the Attorney General over particular types of investigations. The FBI usually is the lead in public corruption cases involving elected government officials. In another instance, HSI is normally the lead agency in matters with a clear nexus to the U.S. border like drug smuggling, human trafficking or smuggling, and import/export violations.
Many exclusive jurisdictions also exist. In these cases, only one criminal investigative agency is authorized or assigned jurisdiction over a particular Title or type of investigation.
Special agents of the ATF, DEA, FBI, USSS, DSS, USCG, CGIS, EPA Criminal Investigation Division, PFPA, USMS, and ICE/Homeland Security Investigations not only have the power to enforce the General Criminal Code (Title 18), but also applicable state and local laws if authorized by the state in which they are operating. Defense Criminal Investigative Service (DCIS), United States Army Criminal Investigation Command (USACIDC), United States Army Counterintelligence, Naval Criminal Investigative Service (NCIS), and U.S. Air Force Office of Special Investigations (AFOSI) special agents not only investigate statutes within the United States Code and state and local laws with a nexus to their branch, but are also tasked with enforcing the Uniform Code of Military Justice (UCMJ), a jurisdiction not held by most of the larger federal agencies. All DCIS, USACIDC, Army Counterintelligence, NCIS, and AFOSI special agents enjoy statutory law enforcement authority, although civilian and military agents derive their principal arrest authority from different federal statutes. Other special agents, such as those employed by the National Park Service, have jurisdiction over crimes committed within the boundaries of or have a nexus to the lands managed by their agency or department only. Homeland Security Investigations Special Agents have administrative arrest powers under the Immigration and Nationality Act. They can administratively arrest and detain aliens for violations of United States immigration laws pending federal removal (deportation) proceedings. HSI Special Agents can also seize merchandise and articles introduced into the United States contrary to U.S. laws under Title 19 of the United States Code.
Certain active-duty special agents employed by the military have broad authority, when the nexus of the violation they are investigating occurred on military-controlled areas or there is a military connection.
All of the major Class I railroads, most regional carriers, and some local railways employ their own police departments whose officers carry the title special agent. Railroad special agents are commissioned by the governor or other agency of the state they are employed in, are armed, and carry state arrest powers in all states in which their employing railroad owns property, if authorized by the state. Their primary concern is policing crimes against the railroad, although they sometimes have the authority to police the general public, make arrests on public property, and enforce applicable local, state, or federal laws when necessary.
A state or municipal criminal investigator can also be a special agent if so titled by the employing agency. Some examples include the Alabama Department of Public Safety's Alabama Bureau of Investigation, Illinois Department of Revenue's Bureau of Criminal Investigation, the Georgia Bureau of Investigation, Ohio Attorney General's Office, Iowa Department of Public Safety, Virginia Department of Alcoholic Beverage Control Bureau of Law Enforcement, Tennessee Bureau of Investigation, and the Florida Department of Law Enforcement.
Several Native American tribes also employ special agents as criminal investigators, court investigators, or gaming investigators, and some are deputized as special federal officers of the Bureau of Indian Affairs.
Special Agents, particularly those within the FBI, have been depicted in popular entertainment for years.
The title "assistant special agent in charge" and its acronym "ASAC" are stated frequently throughout the TV series Breaking Bad. For example, in Season 1 through Season 4, both Hank Schrader and Gus Fring are shown to be friendly with Hank's boss, ASAC George Merkert, and in Season 4 Episode 12 "End Times", Steve Gomez tells Dennis the "ASAC" is being pressured to search Gus's laundry for drugs. In Season 5, Hank – who is proud to have been promoted to the job vacated by his ousted boss – repeatedly chants "ASAC Schrader" to his baby niece Holly.
In the TV series White Collar, Peter Burke is the "Special Agent in Charge" (promoted from assistant special agent in charge at the end of Season 6) and the head of the Manhattan White Collar Division.
In the TV series The X Files, the title "special agent" is given to both Fox Mulder and Dana Scully of the Federal Bureau of Investigation. The title is referenced by them numerous times throughout each episode.
The main character from the TV series Twin Peaks, is FBI Special Agent Dale Cooper, and also, other FBI special agents make numerous cameos throughout the series, and it’s prequel movie, Twin Peaks: Fire Walk With Me.