Mercy Street by Mariah Stewart

Former police detective Mallory Russo is hired as a private investigator when two young people are killed in a city park and their two friends are missing. Mallory, working on a book since resigning her job with the city due to a smear campaign, finds publication doesn't look too promising, and she is anxious to get back into detective work. Her replacement in the Conroy police department, Charlie Wanamaker, is encouraged by police chief Joe Drabyak to assist her as the city has been short-handed due to a random sniper terrorizing the city. Charlie has his own baggage to deal with, nevertheless, he and Mallory click, discovering details that lead to an old, unsolved murder. Plenty of suspense and action. A really good read. Sandy P