The Year of the Knife by G.D. Penman

Agent "Sully" Sullivan is one of the top cops in the Imperial Bureau of Investigation. A veteran witch of the British Empire who isn’t afraid to use her magical skills to crack a case. But Sully might need more than a good education and raw power to stop the string of grisly murders that have been springing up across the American Colonies. Every one of them marked by the same chilling calling card, a threat and a warning in the form of a legion of voices screaming out through the killers' mouths: “It IS tHe YEAr oF the KNife.”