Awaken Me Darkley by Gena Showalter
Part of the Alien Huntres series

It's sometime in the future, and aliens have been residents on Earth for more than 70 years. Enter Mia Snow, head of the New Chicago Police Department's Alien Investigation and Removal Agency, as she and her crack crew try to solve the latest string of extraterrestrially induced murders. This particular killer favors handsome young men with dark hair and dark eyes. Mia knows that the culprit is one of the dreaded Arcadians, the most clever and deadly species of all.

However, when her second-in-command--and best friend--Dallas Gutierrez is mortally wounded, and only Arcadian Kyrin en Arr, the prime murder suspect, holds the key to his survival, Mia has to make a hard choice between her ethics as a cop and her love for her friend. To make matters worse, she is physically attracted to Kyrin, and suddenly, her black-and-white world has gone all swirly gray.

Interesting book and it might be a series eventually from what the author wrote at the end of the book. It was a good mix of Buffy the Vampire slayer and erotica.