Death Stalker: The Poison ID Unit. The Poison ID Unit, #2

FBI agent Lance Knight specializes in identifying obscure toxins. But when victims show up carved with sinister calling cards, he's thrust into a cat-and-mouse game against a diabolical serial killer called The Master. Lance's obsession with finally defeating this enemy from his past threatens to destroy his career and his team. Every twist leads Lance further down a tunnel of paranoia and mind games designed specifically to torment him.
This adversary seems to see all and know all. But unknown to Lance, the greatest poison is within the deception itself. With lives on the line, Lance must separate truth from the web of lies before a brilliantly sinister plot consumes everything he holds dear.