Chasing Darkness by Danielle Girard

The most dangerous place to hide is in the dark...
A tough special-agent with the Department of Justice, and a dedicated single mother to her orphaned nephews, Samantha Chase is unbreakable. Until two women, both from the shadows of Sam's past, are discovered strangled. For Sam, the most unnerving detail is the eucalyptus branch tucked behind the victims' ears—the signature of a notorious serial murder already convicted and executed for his crimes.

To detective Nick Thomas, the only link to the copycat killings is Sam herself. And when she becomes the target of threats—each one more menacing than the last—his feelings for her just might jeopardize the case. But to understand the killer's twisted motive is to explore Sam's past—and a crime she's spent a lifetime trying to bury...