Marked By the Guardian by Kathy Lyons

She investigates murders. He controls monsters.

Mark Robertson’s fortress of locks and isolation isn’t keeping the world out—it’s keeping his bear caged. After what the scientists did to him, he can’t trust the beast inside. Especially not around Detective Julie, whose fearless eyes see past every wall he’s built.
Julie needs Mark’s computer genius to catch a killer. She doesn’t expect a man whose every breath is a battle for control—or the fierce protector hiding beneath his damage.
When the investigation exposes mad scientists experimenting on shifters, Mark’s nightmare becomes real. His tortured brother seeks vengeance. The enemy wants them both dead. And Mark’s control shatters the moment Julie’s in danger.

He needs to believe he’s not a monster.
She needs someone strong enough to stand in the darkness with her.
Together, they’ll tear down the conspiracy—if Mark’s beast doesn’t destroy them first.

Originally published as License to Shift