The Devil's Punchbowl by Greg Iles

This is the third book in a set featuring the fictional character Penn Cage. Penn has won the special election for mayor, but is disappointed with his limited ability to change the city he so loves. When an old friend Tim Jessup comes to Penn with information about local illegal activities, he promises Penn to bring him evidence, but before Tim can produce the evidence to Penn, Tim is tortured and killed. Tim's boss, under the impression Tim was able to get the evidence to Penn, confront Penn and threatens him. Restricted by his position as Mayor, Penn does his best to solve this mystery and rid his city of these illegal activities.