Federal Bureau of Investigation

The Federal Bureau of Investigation (FBI), or Federal Investigation Bureau (FIB) in Grand Theft Auto IV, is based off the real life FBI law enforcement agency.

GTA III — GTA Vice City Stories
The FBI is the third level of law enforcement that pursues the player. The FBI will appear only when the player's wanted level reaches five stars, before the Army on six stars.

FBI Agents wear blue jackets with "FBI" printed on the back in nearly all of the GTA III era games (Excluding GTA III itself). In Grand Theft Auto: Vice City Stories they wear black vests over gray t-shirts with "FBI" printed on the back with dark glasses. The FBI agents are well armed in all appearances, and their vehicles are generally extremely fast. The FBI comes four agents in each car, and since they are well armed and very accurate, they will kill the player extremely quick if they have an open range. They carry MP5 in all of the GTA III era games except for Grand Theft Auto III where this weapon is unavailable and instead they carry AK-47's and will also drop pistols if killed.

GTA IV — GTA Chinatown Wars
In GTA IV, the agency is known as the Federal Investigation Bureau (FIB), presumably to avoid conflict with the real-life FBI. The name is also a play on words, since fib is a synonym of lie.

As the wanted level system changed in GTA IV, they now appear at four stars, sometimes along with NOOSE to assist the LCPD, and more heavily at five and six, and will expertly drive and doggedly follow the player. They now drive FIB Buffaloes with four agents to a car and will either shoot at the player out their windows or try to spin the player out if the player is in a car. FIB Agents with their SMGs can make short work of a player's getaway vehicle if he/she doesn't outrun or take out the FIB Buffalo(es) quickly. On foot, agents are typically armed with Carbine Rifles, although some agents carry Pump Shotguns and SMGs instead. Additionally in The Ballad of Gay Tony, agents carry the Automatic Shotgun and the Assault SMG. FIB agents appear to be more accurate than NOOSE's Tactical Response Unit; however, FIB agents are more susceptible to return fire, making them more likely to drop their weapons when shot.

Unlike the LCPD and NOOSE, both of which employ young to middle-aged Caucasian, African American, and Hispanic officers, the FIB is solely employed by late middle-aged Caucasian men. They use different types of uniforms; some agents wear random clothes with navy blue jackets jackets with "FIB" printed in black letters on their backs with an eagle design. Other agents wear random clothes with their distinctive blue track jackets with "FIB" printed in black letters on the back and on sleeves. Other agents wear black suit pants with a white shirt and a black combat vest and a badge around their neck reading "FIB". All agents carry their FIB badges on the neck. In GTA IV, when the player reaches a certain level, he/she unlocks a "FIB" hat for their multiplayer avatar. In the TLAD missions Bad Cop Drop and Politics, and in the TBOGT mission Going Deep, the agents employed are of African American, Caucasian and Asian descent, and wear different apparel, such as suits and ties, overcoats, blue FIB track coats, and others who where a tactical vest with the letters "FIB" printed in yellow on the front and the back.

In GTA: Chinatown Wars, the agency is also named FIB. After gaining 4 stars wanted level, the FIB will be dispatched to the area. Instead of driving an FIB Buffaloes, they instead drive a black FIB Rancher (which looks like the Cavalcade with modified headlights and grilles). Despite law enforcement vehicles don't usually skillfully chase the player, or even follow at all at high speeds, the FIB can be easily evaded due to their slow FIB Ranchers. However, unless the player get rid of the wanted stars, there will be just more FIB Ranchers spawning in front after they're evaded.

On foot, agents carry Micro SMG (same weapon as NOOSE in GTA: CW). The agents attacks the player similar ways as NOOSE does, except with more gun fire.

Trivia

 * The FBI in GTA Vice City Stories seem to be the most hostile in the GTA III era as their blockades will instantly blow up or heavily damage your car on sight. The best way to manuver through a blockade is to get a bullet-proof car
 * The FBI in GTA III are one of the only law enforcment to wield assault rifles, the other ones being the military at Fort Baxter who have M4s.
 * The FBI vehicle in GTA Vice City Stories is the only one in all of the GTA series to be colored white.