I apologize for asserting that. I didn't read the whole thread, and only picked up your response to my post.
Here is another description of the process, also on the DHS National Response Plan website.
Emphasis on Local ResponseAll incidents are handled at the lowest possible organizational and jurisdictional level. Police, fire, public health and medical, emergency management, and other personnel are responsible for incident management at the local level. For those events that rise to the level of an Incident of National Significance, the Department of Homeland Security provides operational and/or resource coordination for Federal support to on-scene incident command structures.
Proactive Federal Response to Catastrophic Events
The National Response Plan provides mechanisms for expedited and proactive Federal support to ensure critical life-saving assistance and incident containment capabilities are in place to respond quickly and efficiently to catastrophic incidents. These are high-impact, low-probability incidents, including natural disasters and terrorist attacks that result in extraordinary levels of mass casualties, damage, or disruption severely affecting the population, infrastructure, environment, economy, national morale, and/or government functions.
I note the use of the word "support" in "expedited and proactive Federal support," and not the word "takeover."
http://www.fema.gov/library/stafact.shtm <- Very dry reading