The Ghost Cell is the bad guys, the villains,of the whole book series. Made up of an unknown amount of people, so deeply embeded in the US that most doubt their very existence. Made up of plenty of people, where the people barely know the other people, and they can blend in most anywhere, even the White House.
We do know some things about the Ghost Cell. It is run by five leaders, with a hierachry among them. Number one is Peter "Speed" Paulson. The other four change throughout the books as they get killed and Malak (a spy inside) rises throught the ranks.
We also know a bit about some of hte people in the Ghost Cell.
The number one, the leader of the Ghost Cell.
The spy that they have inside.
Malak's twin sister.
Anmar's foster mom.
Anmar's handler.