A two-fisted thriller throws several one-two punches but flags to a lazy last round. Former NYPD detective Brian McKenna lives the good life in Ft. Myers, Fla., far from the city's crowded mean streets. In fact, he lives someone else's life, having taken on a new identity after his last big score (Detective First Grade, 1993). Naturally, though, Brian can't stay retired long, since (ho-hum) police work is in his blood. Here, he's drawn back into service by his pal Ray Brunette, whose cop son was mowed down in a highly suspicious ``robbery.'' Could this brutal killing have been a revenge plot against both McKenna and Brunette? You bet. It doesn't take McKenna long to get himself involved up to his neck with Senderistas, terrorists based in Peru--though that geographical fact doesn't forestall bloodshed all over the hemisphere. McKenna's the man to wrap it all up, even though he needs the help of the beautiful Elena, who falls for him. Mahoney launches the novel with a riveting setup, but settles into a monotonous rhythm: One violent scene follows too quickly on another, then long stretches of the text just talk themselves out. The villains are indistinguishable, meanwhile, and McKenna himself could have stumbled out of hundreds of other books. Great dialogue, though. With guidance, the author could become a real page-turning contender in the police thriller genre.