Cross the line by James Patterson 

Shots ring out in the early hours in the suburbs of Washington, D.C. When the smoke clears, a cop lies dead. Under pressure from the mayor, Alex Ross steps into the leadership vacuum to investigate the audacious killing. But before Cross can make any headway, a wave of muders erupts across the city. The victims have one thing in common – they are all criminals.

The only thing more dangerous than a murderer without a cons-cience is a killer with justice on his side.