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.
Team Bartowski broke in to obtain a list of Fulcrum agents, which Guy had on an encrypted flash drive 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 eventually killed by a Fulcrum agent who breaks into the suite.