Changelings / Jo Bannister.

Enhanced descriptions from Syndetics:

The town of Castlemere is under siege. Someone is making very dangerous-and convincing-threats to destroy the city if it doesn't come up with a ransom of one million pounds. Now time is Castlemere's enemy-it seems the town's bells are tolling not for the passage of time but of people.

With the casualty rate rising, the pressure is on Detective Superintendent Frank Shapiro to catch the blackmailing killer. But the mastermind slips through his fingers just as these precious hours slip through Castlemere's. Along with her professional troubles, Detective Inspector Liz Graham finds she has problems of her own. Panic in the town is at the boiling point when someone suggests her gentle art teacher husband is to blame, and her world begins to crumble. To make matters worse, her sergeant, Cal Donovan, disappears from his autumnal cruise into the watery wastes of the Castlemere Levels. Cal's deserted boat has been located near the tine village of East Beekham, a flyspeck on the map where no one is quite what he seems.

Then again, since the gentle have turned violent and the peaceful turned accusatory, nothing in the entire region is what it seems...