The Fourth Perimeter by Tim Green
Former secret service agent turned successful entrepreneur Kurt Ford thought he had suffered his share of tragedy when the wife he adored died, leaving him to raise their young son alone. But years later, his beloved son - who also became a secret service agent - turns up dead, and Ford is inconsolable. He refuses to accept the government's story that his son committed suicide and begins investigating the case as a murder. Ford soon learns from an old secret service friend that two other young agents have also recently met with unusual deaths, and he discovers a common link between these agents and his son: all three were involved in a cryptic meeting with the president. If this meeting proves to be the cause of his son's death, Ford will attempt to do what only four other men in history have been able to accomplish - he will kill the president of the United States.