The Sigma protocol

Department of Justice field agent Anna Navarro is being stalked around the world by a relentless killer, managing to survive the killer's attacks only by a combination of luck, skill and her own quick wits. These attacks are somehow related to her current assignment: investigating the sudden - and seemingly unrelated - deaths of a number of very old men throughout the world. The only thing that connects them is a file in the CIA archives, over a half-century old, marked with the same puzzling code word: SIGMA. But someone or something is always seemingly one step ahead of her, the survivors are rapidly dwindling, and her own life is in ever increasing danger.