Compared to the CIA, FBI is enormous. When FBI personnel deployed to investigate East Africa bombings, they deployed more agents than the CIA had operations officers worldwide. FBI has regulations about sending two agents to do almost any task. CIA officers usually operate alone – and almost always alone in the development and management of human sources.
While both CIA and FBI put a premium on a good source, the FBI rarely pursues them beyond the context of an investigation. Agents follow leads and seek cooperating witnesses or informers who are often compelled to cooperate or face legal consequences.
FBI (and DEA) often prefer to arrest a suspect rather than ‘turning them’ to take down a network of criminals. And even when they’re turned, if they’re percipient witnesses, they must be disclosed in court proceedings do they don’t remain in place and productive for long. The FBI wants evidence and testimony from witnesses that leads to convictions and (sometimes more importantly), press conferences.
In short almost everything that the FBI does is precisely counter to the requirements of an intelligence agency – from the endless courting of the press, the parade of agents into the Halls of Congress and the simple process of ‘justice’. An intelligence agency is not about being just, it’s about being accurate and there is a distinct Constitutional difference between the two.