In the United States, Special Agent is usually the title of a detective or investigator for a state, county, municipal, federal or tribal government who primarily serve in investigatory roles. Uniquely, many railroad police departments in the U.S. utilize "Special Agent" as either a rank or title, even if the special agent's role is not primarily investigatory in nature. Within the United States' federal law enforcement system there are dozens of federal agencies that employ federal law enforcement officers, each with different criteria pertaining to the use of the titles "Special Agent" versus "Agent." In general, some, but not all, agents are federal law enforcement officers, and hold either arrest authority or the right to conduct minor criminal/non-criminal investigations. However, there are some agencies where 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, whereas only some agents are authorized to do so.