Second Honeymoon by James Patterson

A newlywed couple steps into the sauna in their deluxe honeymoon suite-and never steps out again. When another couple is killed while boarding their honeymoon flight to Rome, it becomes clear that someone is targeting honeymooners, and it's anyone's guess which happy couple is next on the list. FBI Agent John O'Hara is deep into solving the case, while Special Agent Sarah Brubaker is hunting another ingenious serial killer, whose victims all have one chilling thing in common.

As wedding hysteria rises to a frightening new level, John and Sarah work ever more closely together in a frantic attempt to decipher the logic behind two rampages. From "The Man Who Can't Miss," Second Honeymoon is the most mesmerizing, most exciting, and most surprising thriller ever (Time).