The Federal Bureau of Investigation (FBI) is an internal United States government body involved in both criminal investigations and intelligence gathering domestically. The Bureau was founded in 1908, and currently has over 30,000 employees.
We first see the FBI in "Chuck Versus the Fat Lady", when Team Bartowski are breaking into the hotel room of the deceased scientist Guy Lafleur, which is under strict FBI guard. Team Bartowski commit B&E to obtain a list of Fulcrum agents, which Guy has on an encrypted flash drive which they find inside a puzzle box that Chuck solves. Casey doesn't seem to have a lot of respect for members of the FBI, as he comments, "It's the FBI. They're dumb, not deaf", when Chuck makes a lot of nose by falling out of the air vent. The agents guarding the doors are all later killed by a Fulcrum agent who breaks into the suite.