Dexter: "Looks like they want to take over the case.."
Debra: "It wouldn't fucking surprise me, it's what the feds do best"
The Federal Bureau of Investigation (or in short, the FBI) is an organization in DEXTER. The FBI is a highly trained government-based law enforcement agency that specializes in terrorism and other National Threats. In retrospect, they only appear when a case becomes significantly large, such as the Bay Harbor Butcher Investigation and the Trinity Case. Due to their involvement in cases, they are often seen as having somewhat of a "rivalry" with the local law enforcement, in this case the Miami Metro Police Department and once they become involved in an investigation, the outcome is almost always credited specifically to the FBI despite the groundwork laid out by the police.
An interesting fact is that the FBI has always become involved in each of the "even" Seasons of DEXTER and has become somewhat of a trend in the series. While they do appear in Season Five, it is merely at the beginning of the season wrapping up the events of the previous Season's Trinity Case as well as the Rita Morgan Crime Scene. They have again made their appearance in Season Six, much to the dismay of local detectives such as Joey Quinn who believes they will take all of the credit in the Doomsday Killer Investigation.