Next Victim by Michael Prescott

Tess McCallum is a blonde, gray'?�eyed, single, and totally dedicated FBI agent whose work and personal life have both been stuck in neutral since the traumatic night she came home to find her fellow FBI agent and secret lover ritually murdered by the slippery sex killer she had been pursuing. Her target, Mobius, is a crafty, complex, and completely insane serial killer. His unique scariness lies in his ordinariness; he is an Everyman who could be just about anyone . . . or anywhere. Now, two unnervingly inactive years later, Tess gets a summons from her former boss, Assistant Director Gerald Andrus, bringing her to L.A. to investigate a suspect who seems frighteningly similar to Mobius. He is back, with a new identity and, as a result of his latest opportunistic killing, a new weapon of mass destruction (a canister of VX nerve agent), as well as a nasty plan to kill thousands all at once. Tess must unravel the puzzle and figure out the secret of Mobius before he kills her, along with a big chunk of Los Angeles. The suspense starts early and does not quit.

This book is part of the "Abby Sinclair and Tess McCallum" series.
Here are some other books from this series: