Operation: Firewall was the brainchild of Dennis Ryland and other major figures in Washington. It was designed as a way of "protecting" the world from the 4400. It involved injecting all the 4400's with the promicin-inhibitor which prevented the returnees from using their abilities and in many cases suppressing the development of abilities all together.
Effects on the 4400 Edit
The 4400 began experiencing side effects of the inhibitor during season 2. The side effects resembled effects of diseases. Because of this all the sick 4400's were instructed by the government to go into quarantine. Twenty-eight 4400's died from the effects of the inhibitor.