The First Commandment (Scot Harvath #6) by Brad Thor

Covert counterterrorism agent Scot Harvath awakens find a sadistic assassin with a personal vendetta unleashes horrors on those closest to him. Ordered by the president to stay out of the investigation, Harvath mounts his own operation to uncover the conspiracy and exact revenge. When he discovers a connection between the attacks and a group of prisoners secretly released from Guantanamo, Harvath must ask himself previously unthinkable questions about the organizations and the nation he has spent his life serving.