He is part of the US Army's Delta Force. In Crucifixion: Voodoo Plague Book Two he mentions a flashback to running through a Central American jungle with Spetsnaz in pursuit. This is that story. Note that this is a short story and does not need to be read in any particular sequence with the Voodoo Plague series. All events are prior to the attacks and events that occur in the series and this will not spoil that timeline.