Dirty magic / Jaye Wells.

Enhanced descriptions from Syndetics:

Action-packed police procedural meets dark urban fantasy in a world where cops and wizards are fighting a war over addictive, dangerous, and illegal dirty magic.

Magic is a drug. Careful how you use it.

The last thing patrol cop Kate Prospero expected to find on her nightly rounds was a werewolf covered in the blood of his latest victim. But then, she also didn't expect that shooting him would land her in the crosshairs of the Magic Enforcement Agency, who want to know why she killed their lead snitch.

The MEA keeps dirty magic off the streets, but there's a new blend out there that's as deadly as it is elusive. The more Prospero learns about the investigation, the more she's convinced that earning a spot on their task force is the career break she's been wanting. But getting the assignment proves much easier than solving the case. Especially once the investigation reveals their lead suspect is the man she walked away from ten years earlier-on the same day she swore she'd given up dirty magic for good.

Kate Prospero's about to learn the hard way that crossing a wizard will always get you burned, and that when it comes to magic, you should never say never.

The first book the Prospero's War series from bestselling author Jaye Wells.