Frontier Vengeance by Michael Cardwell

Danny Coogan is now working as a US Marshal, and he is within weeks of becoming a father for the first time. During a takedown of a fugitive, he makes a grisly discovery: a body, and it's the son of someone he knows and works with. Thus begins an off-the-books investigation that leads him to millions of dollars in stolen money, an entire collection of dead bodies, and an ultimate showdown with an adversary determined not to be taken alive.