The First Rule by Robert Crais

Frank Meyer had the American dream - until the day a professional crew invaded his home and murdered everyone inside. The only thing out of the ordinary about Meyer was that - before the family and the business and the normal life - a younger Frank Meyer had worked as a professional mercenary, with a man named Joe Pike. The police think Meyer was hiding something very bad, but Pike does not. With the help of Cole, he sets out on a hunt of his own - an investigation that quickly entangles them both in a web of ancient grudges, blood ties, blackmail, vengeance, double crosses, and cutthroat criminality, and at the heart of it, an act so terrible even Pike and Cole have no way to measure it. Sometimes, the past is never dead. It's not even past.