DEEP BLACK CONSPIRACY 2008 Deep Black series 6 by Stephen Coonts and Jim DeFelice

A Secret Service agent is dead, an apparent suicide. A presidential candidate narrowly escapes an assassin's bullet. And Desk Three, a convert branch of the NSA, is searching for a chilling connection deep inside The Republic of Vietnam.
Once, Charlie Dean was a Marine sniper in Quang Nam Province. Today he's a Deep Black operator, returning to Vietnam to find the source of some threatening e-mails. Instead, he comes face to face with a man he had once hunted down and thought he had killed.
Back in the U.S., Deep Black agent Lia DeFrancesca has uncovered the trail of a killer in Dean's path. Now, with every asset, weapon, bug and high-tech magic wand Desk Three can wave, the agents enter a terrifying global race against time. Because ghosts of the past have risen to life to strike a death blow into the heart of the U.S.A.