CENTCOM believes that  the Iraqi resistance consists of 8-12 separate groups, each with about a hundred core members and "attack teams" of 10-20 men who plan and carry out the attacks. There appears to be some coordination, if only in the sense that the attacks come in waves. So it's either a "me-to" effect or prior agreement to go out and arrange attacks for a certain date. The leaders and members of the resistance appear to be former members of Saddams intelligence and police forces and Republican Guard. The U.S. has had criminal investigators building cases against Iraqis by collecting and examining evidence. This has led to more arrests, as additional people are connected to attacks.