Hostile Intent, by Michael Walsh, was previously free in Aug 2009.
Book Description
It starts with the unthinkable--the most horrific act of violence ever committed on American soil.
Only one man can stop them.
Code named Devlin, he exists in the blackest shadows of the United States government--operating off the grid as the NSA's top agent. He's their most lethal weapon-and their most secret. But someone is trying to draw him out into the open by putting America's citizens in the crosshairs--and they will continue the slaughter until they get what they want.