Agents from the Federal Bureau of Investigation in the United States are referred to as FBI agents for short!
This organization is indeed well-known in later times, but the FBI was formally established in 1908.
In its early days, its main responsibility was to investigate social malaise within the United States, such as the mafia, gambling, brothels, etc.
What's most important is that the FBI is under the jurisdiction of the National Government, which effectively increased the National Government's law enforcement power, especially since before this, states in America had a high degree of autonomous rights, and often some states would defy certain commands from the National Government.
The most obvious example is that Atlantic City did not enforce Prohibition.
